The Journal of San Diego History
Spring/Summer 2000, Volume 46, Numbers 2 & 3
Gregg Hennessey, Editor

By Jorge A. Vargas

Photographs from this article

The Treaty of Guadalupe Hidalgo is the bilateral instrument which first established the international boundaries between Mexico and the United States. Almost since 1848, when this treaty was signed, voices in Mexico have claimed that this legal instrument gave the United States more territory than it was intended pursuant to its Article V, which established the limits between both nations. For example, it has been alleged that the separating line between the state of California, U.S.A. and Baja California, Mexico, is placed farther due South than it should be.1 This article attempts to provide a legal answer to this old but still intriguing question.

The establishment and demarcation of international boundaries are among the most important and delicate questions in international law. As is known, boundaries play a crucial role in the bilateral relations between States because they define those specific areas — territorial, fluvial, oceanic, submarine and aerial — under the sovereignty of a given nation. Whereas in the past boundary questions principally centered on land and river matters, the advent of scientific and technological developments in recent decades have expanded and also diversified the notion of boundaries. Today, all countries realize the technical complexity associated with international limits and with their political and, especially, their economic implications.

Boundaries are established by international agreements between nations “formally known as treaties” and delimit not only that traditional portion of the land mass upon which each individual nation is located but also other contiguous physical spaces, such as rivers, or more modern spaces such as oceanic areas, submarine regions, the seabed and ocean floor and even the air space above the territory of a given State.

In the case of Mexico, the history of its territorial boundaries with the United States and the chronology of the admirable and almost heroic events that led to the final establishment of these limits pursuant to the Treaty of Guadalupe Hidalgo and the Gadsden Purchase of 1853 is a story still waiting to be properly narrated and analyzed. Notwithstanding that 152 years have already elapsed since the signing of the first of these two treaties, Mexico’s legal, diplomatic, historic and technical literature on this fascinating subject remains scarce.2

Mexico and the United States share one of the oldest, longest, and most complex boundaries in this hemisphere. According to the International Boundary and Water Commission (IBWC), the “official” length of this bilateral limit totals 1,951.36 miles, divided this way: 1,253.69 miles of the Rio Grande; 697.67 miles from El Paso, Texas, to the Pacific Ocean, and 23.72 miles of the short stretch of the Colorado River.3

From the viewpoint of international law, this binational limit consists of two different types of “boundaries:” 1) “Natural boundaries” which are those formed by rivers and mountains, such as the Rio Grande, the Colorado and the Tijuana; and, 2) “Artificial boundaries” consisting of straight lines connecting the Rio Bravo with the Pacific Ocean that follow certain parallels and meridians in degrees of latitude and longitude. These artificial lines were also marked astronomically. Examples of these boundaries are those running west from Ciudad Juarez, Chihuahua, and El Paso, Texas, to the Pacific Ocean.4

Although the original intention of the plenipotentiaries who negotiated these boundaries was to consider them (including the natural boundaries) as “fixed and definite,” i.e., eternal, the sudden changes and gradual movements of the international rivers in question, subject to natural phenomena such as floods and droughts, for example, proved them to be wrong. Eventually, this resulted in the signing of a bilateral convention in 1884 which established the rules applicable to the movements of these rivers. The implementation of the rules enunciated in this convention necessitated the establishment of a binational body to do it. This was to be the International Boundary Commission (IBC), created in 1889,5 whose functions were expanded in 1944 to include jurisdiction over the waters of the international rivers.6 Currently, the International Boundary and Water Commission (IBWC), composed of a U.S. and a Mexican section headed by a respective Commissioner, is empowered to decide matters affecting boundary questions as well as the utilization and allocation of the waters of international rivers.

Article V of the Treaty of Guadalupe Hidalgo7 established the boundary between both countries. The article reads:


The boundary line between the two Republics shall commence in the Gulf of Mexico, three leagues from land, opposite the mouth of the Rio Grande, otherwise called Rio Bravo del Norte…; from thence, up the middle of that river… to the point where it strikes the southern boundary of New Mexico (which runs north of the town called El Paso) to its western termination; thence, northward, along the western line of New Mexico, until it intersects the first branch of the River Gila;… thence down the middle of the said branch and of the said river, until it empties into the Rio Colorado; thence, across the Colorado, following the division line between Upper and Lower California, to the Pacific Ocean.8

While negotiating the treaty, the U.S. and Mexican plenipotentiaries confronted a serious problem: they did not know where “the division line” between Upper and Lower California was. This was mainly due to the remoteness and inclemencies of that area, where relatively few explorers had ventured to survey it prior to 1848, and to the lack of accurate maps depicting said limit.9

The determination of the Californias’ boundary was a particularly crucial issue for the United States. Upon this decision depended whether the port and the bay of San Diego would be included as part of the territories to be “ceded” by Mexico to the United States, pursuant to the Guadalupe Hidalgo Treaty. In addition, the U.S. had already made plans to take advantage of its military victory over Mexico to acquire certain territories which would be used to build a much needed transcontinental railroad across the southwest of the United States.10

To solve this problem, the plenipotentiaries of the Guadalupe Hidalgo Treaty utilized three strategies: first, they included a specific provision in the Treaty agreeing to the limit between the two Californias; second, they annexed to the Treaty a copy of an old Spanish map sketched in 1782 by the Second Pilot of the Spanish Armada, Juan Pantoja y Arriaga known as “the Pantoja Map” which the plenipotentiaries used to depict the newly agreed California limit; and third, they empowered the government officials who were later appointed to establish and demarcate the international binational boundary (i.e., one Commissioner and one Surveyor from each country, who formed the original Boundary Joint Commission), with almost absolute powers to resolve by agreement any questions pertaining to said boundary. Furthermore, it was formally stipulated that the agreed results reached by the Commissioners and Surveyors were to have the same force as if they were inserted in the Treaty itself.

The language of the Guadalupe Hidalgo Treaty addressing these three matters reads:


In order to preclude all difficulty in tracing upon the ground the limit separating Upper from Lower California, it is agreed that the said limit shall consist of a straight line, drawn from the middle of the Rio Gila, where it united from the Colorado, to a point of the Pacific Ocean, distant one marine league due south of the southernmost point of the Port of San Diego, according to the plan of said port, made in the year 1782 by Don Juan Pantoja, second sailing master of the Spanish fleet, and published at Madrid in the year 1802, in the Atlas to the voyage of the schooners Sutil and Mexicana: of which plan a copy is hereunto added, signed and sealed by the respective plenipotentiaries.

In order to designate the boundary line with due precision, upon authoritative maps… the two governments shall each appoint a Commissioner and a Surveyor, who… shall meet at the Port of San Diego… They shall keep journals and make out plans of their operations; and the result agreed upon by them, shall be deemed a part of this Treaty, and shall have the same force as if it were inserted therein.11

In essence, this empowering language is one of the special aspects of the 1848 Treaty. Knowing that the physical and topographical data regarding those vast extensions of land where the new boundary was to be established was clearly lacking, or was grossly inaccurate, the Treaty plenipotentiaries empowered the Joint Commission to reach agreement on the precise location of the boundary or on any other aspects pertaining to it. By having these ample powers, the U.S. and Mexican teams had the ability to, say, “adjust” the boundary described in the Treaty to the physical contours imposed by the topography of the land. If the physical features depicted in one of the official maps was incorrect or inaccurate, the Joint Commission had the power to translate the map and treaty language into a concrete reality, into an agreed physical boundary marked with monuments upon the land.

Another unique aspect of this empowering language was the formal understanding that any boundary line expressly agreed by the two sections of the Joint Commission formally became the official international boundary, de jure and de facto; and this legally binding language, as a consequence, was deemed to be part of the Treaty itself. Thus, each important decision regarding the location of the boundary was couched in terms of an agreement, i.e., a legally binding contract. By adopting this modus operandi the boundary agreed by the Joint Commission became final and definite, subject to no ulterior changes.

The Boundary Joint Commission convened for the first time at San Diego, California, on July 6, 1849.12 The Mexican government appointed Gen. Pedro Garcia Conde, as Commissioner, and Jose Salazar Ilarregui, as Surveyor; and the U.S., John B. Weller and Andrew B. Gray, respectively. Three days later, the Commission agreed to conduct surveys and define on the ground: (1) the southernmost point of the port of San Diego, and (2) the two extreme points of the straight line between the Pacific and the junction of the rivers Gila and Colorado.

The “Initial Point” on the Pacific Ocean, situated one marine league due south of the southernmost point of the Port of San Diego, according to the Pantoja Map, was officially ascertained on October 10, 1849. A written statement in English and Spanish was placed in a bottle hermetically sealed and deposited in the ground and signed by the U.S. and Mexican Commissioners and Surveyors in the presence of two witnesses.13 It was agreed that this point was in North latitude 32o 31′ 59″ and the longitude thereof 117o 48′ 21″ west of Greenwich. On January 30, 1850 the Joint Commission agreed to place a monument at the Initial Point on the Pacific Ocean.

The major argument advanced by Mexican authors14 to claim that this portion of the boundary is placed further due south than it should be is based in the interpretation of the words “port” and “bay.” As seen earlier, the Treaty reads that the Initial Point should be placed “one marine league due south of the southernmost point of the Port of San Diego.” However, the official reports of the Joint Commission clearly indicate that the marine league was measured not from the “port” of San Diego “which was at that time located some 9.5 miles north” but from the lowest coastline of the Bay of San Diego.

According to this argument, the boundary should have been measured from the then “port” of San Diego, which in 1848-1850 was located in the area known today as Ballast Point. This Point was labeled “Punta Guijarros” on the Pantoja Map and is currently located in the inland area of Point Loma, slightly northeast from the Cabrillo National Monument and across from the North Island Naval Air Station in Coronado. Ballast Point and the top of Coronado configure the mouth of the channel used by vessels to enter into the Bay of San Diego from the Pacific Ocean today.

The difference between the Initial Point measured from the “port” than measured from the lowest coastline of the “bay” consists of some 9.5 miles. Now, when one considers that the straight line that separates California, U.S.A. from Baja California, Mexico, has a length of 146.9 miles to its Eastern Terminus15 (in the confluence of the Rivers Gila and Colorado), the territorial loss affecting Mexico seems to be considerable.

However, legal and technical realities seem to negate this allegation. First, it should be recalled that the 1848 Treaty conferred almost absolute powers upon the members of the Joint Commission to reach an “agreement” between themselves on the location of the boundary. Accordingly, as documented in the official minutes of the Joint Commission, dated October 10, 1849, the Mexican and the U.S. members of the Commission “agreed” on the location of the Initial Point of the boundary on the Pacific Ocean.16

Second, there is even a stronger argument to dispose of this claim. The precise location of the current international boundary between both countries was marked, in red ink, on the Pantoja Map, at the time when this map was added to the Guadalupe Hidalgo Treaty.17 Furthermore, in order to avoid any future discrepancy regarding the location of this portion of the boundary “especially from the U.S. perspective” the Pantoja Map was signed by each of the three Mexican plenipotentiaries: Bernardo Couto, Miguel Atristain and Luis G. Cuevas, and by Nicholas P. Trist, the U.S. Commissioner, and an official seal was affixed to it. Thus, the U.S. and Mexican plenipotentiaries not only indicated that this was the map referred to in Article V of said Treaty but, more importantly, expressly recognized that the line in the San Diego-Tijuana region had been personally drawn in red ink as the agreed “boundary line” (Linea divisoria) between both countries. Accordingly, the technical work of the Joint Commission merely consisted in tracing upon the ground the boundary line depicted in red in the Pantoja Map.

In closing, it should be acknowledged that during the slow and technically challenging process of establishing and demarcating the binational boundary by the Joint Commission, an admirable joint effort that took from 1849 until 1857, many technical mistakes were made. Most of them were due to mechanical defects affecting the operation and accuracy of the scientific instruments utilized by the Commission. When these mistakes or the resulting technical inaccuracies went beyond the reasonable standard agreed by the Commission, they were immediately brought to the attention of the other party and, when deemed necessary, corrected. However, when the discrepancies of the technical readings between the Mexican and the U.S. sections were considered to be tolerable, given the hostile environmental and technical working conditions when the boundary was being established, these discrepancies were solved by agreement between the two sections.

In the relatively few cases where one of the contracting parties considered that a gross misreading or a gross mistake had taken place, the affected party submitted the case to the International Boundary Commission (IBC) or even to the International Boundary and Water Commission (IBWC) for its technical analysis and final decision. In this regard, for example, Mexico brought attention to a number of important cases, including Rancho de Sasabe, Mina Oro Blanco, La Tinaja, Tres Bellotas, La Noria, El Durazno (in Sonora), Ascencion (Chihuahua), etc.18 However, no formal claim has ever been submitted by Mexico regarding the Initial Point on the Pacific Ocean or the Eastern terminus at the confluence of the Gila and Colorado Rivers, or any other point along the straight line that separates California, U.S.A., from Baja California, Mexico.

The fundamental principle that has guided, and continues to guide, the work of the IBWC is that the boundary established by Article V of the Treaty of Guadalupe Hidalgo of 1848 and Article I of the Gadsden Purchase of 1853 is to be respected by each of the two countries, and that no change shall ever be made to the international boundary, “except by the express and free consent of both nations, lawfully given by the General Governments of each, in conformity with its own Constitution.”19 There is no question that this lucid and sound international law principle, established one hundred and fifty-two years ago, will continue to be in force for many years to come.



1. Another claim advances the thesis that the California Channel Islands (formed by eight small islands offshore California) continue to belong to Mexico. See Jorge A. Vargas. El Archipielago del Norte. Territorio de Mexico o de los Estados Unidos? Fondo de Cultura Economica, Mexico, 1993 and Loyola of Los Angeles Internationa & Comparative Law Journal “California’s Offshore Islands: Is the ‘Northern Archipielago’ A Subject for International Law or Political Rhetoric?” Vol. 12, May 1990, No. 3 at 687-724.

2. Traditional sources include Cesar Sepulveda. La Frontera Norte de Mexico. Historia y Conflictos, 1762-1975. Porrua, Mexico, 1976; Alberto Maria Carreno. Mexico y los Estados Unidos de America. Jus, Mexico, 1962; and, Luis G. Zorrilla. Historia de las Relaciones entre Mexico y los Estados Unidos de America, 1800-1958. Porrua, Mexico, 1977. Analytical works discussing Mexico’s territorial or maritime boundaries with other countries, such as Guatemala, Belize, etc. appear to be simply non-existent.

3. Leon C. Metz. Border. Mangan Books, El Paso, Texas, 2nd ed., 1990, no pagination.

4. Oceanic and submarine boundaries, such as the territorial sea, the continental shelf and, more recently, the Exclusive economic zone (EEZ), may be traced based on the straight-baseline system, following agreed points of latitude and longitude. See, for example, the Treaty on the Delimitation of Maritime Boundaries between Mexico and the United States, signed in Mexico City on May 4, 1978.

5. Convention to Avoid the Difficulties occasioned by Reason of the Changes which take place in the Beds of the Rio Grande and the Colorado River. Signed at Washington March 1, 1889; entered into force on December 24, 1890. 26 Stat. 1512; TS 232; 9 Bevans 877.

6. Treaty relating to the Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande, and Supplementary Protocol signed on Novermeber 14, 1944. Signed at Washington D.C., February 3, 1944; entered into force November 8, 1945. See 59 Stat. 1219; TS 994; 9 Bevans 1166; 3 UNTS 313.

7. Treaty of Guadalupe Hidalgo. Treaty of Peace, Friendship, Limits, and Settlement, signed at Guadalupe Hidalgo on February 2, 1848. Ratified by the United States March 16, 1848. Ratified by Mexico May 30, 1848. Ratifications exchanged at Queretaro, Mexico, May 30, 1848. Proclaimed July 4, 1848. Treaty Series No. 207. Treaties and Other International Acts of the United States of America. Edited by Hunter Miller. Vol. 5, Document 129. U.S. Government Printing Office. Washington: 1937 at 204-428.

8. Id. at 213-214. See also Tratados y Convenciones sobre Limites y Aguas entre Mexico y los Estados Unidos. Secretaria de Relaciones Exteriores (SRE). Comision Internacional de Limites y Aguas (CILA), Seccion Mexicana. Ciudad Juarez, Chih., 1957 at 9.

9. This problem, the lack of accurate and technical information regarding the lands where the boundary was to be established, was not limited to the line separating the two Californias but it applied to many other stretches along the long and difficult binational boundary. Mexican and U.S. authors have written extensively on the mistakes and technical inaccuracies that plagued the Pantoja and, especially, the Disturnell maps. These two maps were officially added to the Guadalupe Hidalgo Treaty. See Treaties, supra note 7 at 340, 362-371.

10. Maj. William H. Emory, member of the Joint Commission established in 1849 to demarcate the boundary who later became the U.S. Commissioner, and Andrew B. Gray, U.S. Surveyor of that Commission, were directly involved in gathering technical information to promote the transcontinental railroad project. See, for example, A. Gray. Survey of a Route for the Southern Pacific R.R. on the 82nd Parallel. Southern Pacific Railroad (Railroad Record), 1856.

11. Article V, Treaties, supra note 7 at 215.

12. Report of the Secretary of the Interior, Senate Executive Document No. 119, 32nd Congress, 1st Session (Serial 626) at 56.

13. In part, this statement read: “Be it remembered that, on the 10th day of October, A.D., 1849, the undersigned, Commissioners and Surveyors, duly appointed and commissioned by their respective governments, being satisfied with the results of the survey made, did agree that the demarkation (sic) of the boundary between the United States and the Mexican Republic shall commence at this point….” Id. at 59.

14. See, for example, the pamphlet by Guillermo Ortiz Zamora. The True Border between the Californias. North or South? Port or Bay? Tijuana, B.C., Mexico, 1987.

15. According to its Minute of January 28, 1850, the Boundary Commission agreed that the geographical position of the point Amarking the middle of the Gila River, where it unites the Colorado” was at 32E 43′ 32″ and 7 hours, 38 minutes. Thus, the straight line separating the two Californias, between the momument at the Initial Point on the Pacific Ocean and the confluence of the Gila and Colorado Rivers, has a length of 148.689 miles. However, the U.S. and Geodeitic Survey in 1936 Are-computed” this distance, according the North American Datum of 1927, to be 146.994 miles. See Treaties, supra note 7 at 415, note 1.

16. See supra note 13 and the corresponding text.

17. Please see the language at the end of the first paragraph of Article V of the Guadalupe Hidalgo Treaty, as reproduced in supra note 11.

18. Most of these cases are discussed in Luis G. Zorrilla. Monumentacion de la Frontera Norte en el Siglo XIX. Secretaria de Relaciones Exteriores, Mexico, 1981.

19. Article V, Guadalupe Hidalgo Treaty. Treaties, supra note 7 at 215-216.




Jorge A. Vargas has been a professor of law at the University of San Diego School of Law since 1983. Professor Vargas teaches, writes, and lectures in a variety of areas including Mexican law, NAFTA law, immigration law, and international law. He has law degrees from Mexico’s UNAM School of Law and Yale Law School. His most recent book is, Mexican Law: A Treatise for Legal Practitioners and International Investors, 2 vols., (1998). Contact him at