It was the Texas dispute that provoked the Mexican
Post# of 51169
Americans had settled in Texas, they didn’t assimilate, they became the majority, and seceded from Mexico in 1836.
That was not the first time that Mexico lost territory. Upon independence in 1821, Central America had been part of Mexico, but Mexico lost that territory in the 1820s. Funny, I never hear about a Mexican "reconquista" of Central America.