Many who fell at the Alamo were Mexicans fighting for the 1824 Mexican Constitution —a document patterned upon the American Constitution and one dismissed by General Santa Anna who ruled as a dictator. They were fighting for a free Mexico not Texan Freedom or Independence. These Mexican Republicans are many times forgotten in the story of the battle. It was a clear battle between Republican Freedom and a tyrant dictator. Many who fought called themselves “Texicans” rather than Americans.
I have read that after Santa Anna seized power, many of the Mexican states declared their independence from Mexico.
He then went into Yucatan and pressed many natives into the army and began a slow march north, overthrowing all the independent states until he came Texas.
I believe Great Britain was the catalyst behind the US-Mexico war. They recognized Texas as an independent nation with the Rio Grande as the border, then proposed a treaty between GB-Canada, Mexico and Texas against the United States over the Oregon Territories which was heating up at that time.
This so horrified the Texans that they called to be admitted into the US as a state. The US, realizing the back door was wide open, agreed and troops were sent to the Rio Grande which caused Mexico to start bombardment of the unnamed fort there. The first soldier killed was named BROWN and the fort and city took that name.