If it had been ratified, it would have given major control over Mexican territory that was seen as a crucial transit point from the Caribbean to the Pacific Ocean.
The Mon-Almonte Treaty was signed on 26 September 1859 by Juan N. Almonte, Mexican conservative and Alejandro Mon, representative of Queen Isabella II of Spain, in Mexico.
It even required Mexican troops to assist in the enforcement of the rights permanently granted to the U.S. Additionally, it granted perpetual rights of passage through two strings of Mexican land: one that would run through the state of Sonora from the port of Guaymas on the Gulf of California, to Nogales, on the border with Arizona; and another one from the western port of Mazatlán, in the state of Sinaloa, going through Monterrey all the way through Matamoros, Tamaulipas, south of present-day Brownsville, Texas, on the Gulf of Mexico.
The U.S. hoped to build a railroad or canal across the isthmus to speed transport of mail and trade goods between the eastern and western coasts.
[5] Article 8 granted the United States the right of transporting troops and military supplies across the Republic of Mexico through the Sonora and Tehuantepec Routes and Article 9 granted the United States government the right to protect the aforementioned transport routes by military force, if the Mexican government failed to do so.