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It was all about land my friend ^_^ Most of the western territories belonged to Mexico, such as Texas, New Mexico, California, etc. thats why in California most cities are in spanish like "Los Angeles" translated in English is The Angels. We also had war with Natives just they were much more easier to conqueror from their lack of effective weapons and tech. They had bow and arrows we had cannons and guns.
This war severed ties with the U.S. which used to be good. Mexico also refused to pay many damage claims and other taxes that they would have to pay to the U.S., and that was mainly why it was controversial. Another reason the Mexican American War was so controversial was because many Americans believed that the president's claims that the Mexicans had shot first and wounded American soldiers was a false claim. People believed that they had invaded Mexico without any apparent reason. On the floor of the senate Lincoln claimed that there was no proof that the Mexicans had actually shot at Americans.
Mexican power xD