According to this site they did. https://www.britannica.com/place/Texas-state/Annexation-and-statehood
The only thing I don’t understand about that is, I thought USA and Mexico had a treaty after the Mexican-American war? The Guadalupe Hidalgo treaty.
I believe USA even helped defend Mexico in the French-Mexican war, and the war was ended in 1867 because the French wanted no part of conflict with the US of A. (USA’s military rebounded in 1867 a bit, and US warned France to leave Mexico.)
Anyways, below is a pic of where the site mentions Mexicans attacking Texas during the civil war. Just curious about how that went about, with there being the Treaty of Hidalgo..
During the American Civil War, Mexico itself was engaged in two wars: the War of the Reform up to the end of 1860, and then the Tripartite Intervention and eventual Franco-Mexican War up through 1867. However, I am going to interpret the article as literally as possible:
During the war Texans had to defend themselves . . . from Mexican encroachments”
Juan Nepomuceno Cortina was a Tejano who was born in Texas under Mexican rule and remained there after the American annexation. He like many other Tejanos had experienced abuse under Anglo domination, and in 1859 he started the “Cortina War”. He seized control of Brownsville, shot the sheriff, and started a minor rebellion involving Tejanos vs. Texas Rangers (Weber p. 232). He quickly issued a proclamation explaining the reasons for his actions:
Juan Nepomuceno Cortina to the Inhabitants of the State of Texas, and especially to those of the city of Brownsville . . . Our object, as you have seen, has been to chastise the villainy of our enemies, which heretofore has gone unpunished. These have connived with each other and form, so to speak, a perfidious inquisitorial lodge to persecute and of us, without any cause, and for no other crime on our part than that of our being or Mexican origin . . . (Weber p. 232).
These . .. form . . . a secret conclave . . . for the sole purpose of despoiling the Mexicans of their lands and usurp[ing] them afterwards. This is clearly proven by the conduct of one Adolph Glavecke, who, invested with the character of deputy sheriff . .. has spread terror among the unwary, making them believe that he will hang the Mexicans and burn their ranches . . . (Weber p. 233).
Where this becomes relevant is after heavy pressure from the Texas Rangers he fled to Tamaulipas in Mexico. In 1861 he and his followers reentered Texas to harass the Confederates. After a costly engagement, the Cortinistas withdrew to Mexico and supported Benito Juarez during the war with France. Cortina himself actively favored the Union and in 1863 some of his followers even enlisted in the Union Army after the Union captured Brownsville (UTRGV).
Therefore, while I am not aware of the Mexican government participating in the Civil War, the Confederates did need to defend themselves from encroachments from Mexico.
Sources
University of Texas, Rio Grande Valley. Juan Nepomuceno Cortina and the “Second Cortina War” https://www.utrgv.edu/civilwar-trail/civil-war-trail/cortina-second-war/index.htm
Weber, David. Foreigners in their Native Land: Historical Roots of the Mexican Americans.