Question #f876c

(I have absolutely no clue why it would classify this under "The Gilded Age", as that began approximately in the 1870's. I assume you mean the Mexican-American war of 1846-1848).

There were multiple reasons as to why the Americans declared war on Mexico in this period, but the main reasons are as stated:

A) The U.S. annexation of Texas: Mexico had forewarned the Americans that if they intervened in the Texan affair, there would be serious repercussions. There are distinctions to be made, the Mexicans saw this as a "civil war", but Texas was pretty much a free nation by that point, the "Lone Star Republic", Mexico just said it was a civil war for other nations not to intervene. So when the U.S.A. integrated Texas into the union, Mexico did not immediately declare war, but it severed diplomatic ties.

B) Manifest Destiny: The idea of Manifest Destiny was one that the U.S.A. was ordained by a higher power to conquer the inferior nations. Think of it like Darwinism, in which it's the survival of the fittest. The American eagle, spreading its wings, was a sprawling nation, and they thought they were mandated to spread around from frontier to frontier. This brings into question the fact that James K. Polk (president), wanted to expand into California, due to Manifest Destiny.

C) Boundary Dispute: The U.S.A. argued that the ideal (and therefore righteous) boundary between themselves (Texas) and Mexico was in the Rio Grande (modern day), Mexico argued that the Texan boundary began much north, at the Nueces river.

Conclusion: You can choose any of the 2/3 presented here, in the end, there were multiple justifications, these were the triumvirate of justifications that made up the conquest of Mexico.