Most people did not know that America had actually obtained all this land from Mexico. If we go back to the 1800s, then America was rapidly spreading to the west. During this time, Mexico was trying to strengthen its position. It started to give land to Americans in Texas. The Americans also brought slaves with them, although it was completely illegal according to Mexico's law.
When the Mexican government tried to stop it, the population rebelled. They won the war and called themselves "Public