The Mexican-American war was a conflict between United States and Mexican over whether Texas would end up in Mexican claim or U.S claim.
Why did the war happened?
But why didn't they just lived together as an unite?
well the American weren't following the Mexican laws. They refused to.
What's next?
What happened after Texas got there independence?
2 year after getting there independence they asked the US congress to annex Texas and make them apart of United States.
In the 1830's, Mexico owned all of the southwestern United States including Texas. American stated moving to Texas and the Mexicans that were living there were outnumbered by the American that was living there.
The American settlers refused to follow the Mexican laws. In 1836 American helped Texas fight Mexico to get Texas independent.
Texas wanted to be free from Mexico. They wanted to be apart of Untied States. But US weren't so sure if they wanted Texas because they have a lot of debt to pay and they allowed slavery. But they decided that they wanted it. Mexico was threatening United States war if they annex Texas. But at the end it was US that declared war of Mexico because they desired to retain the disputed land of Texas and obtain more of Mexico's northern lands.