Cause they hd to adopt to a new kind of religion to faith because most Mexicans did not pledge for an allegiance to Spain cause Spain does not want to have an alliance with any Mexicans. But with the Native Americans did not want to be friendly because they protect their lands so they nothing with Mexico and they don't grow Cotten in the 1800's most Cotten farms do not come from Mexico.
You haven't given a list of options but ill list off some reasons, #1 The U.S explained that they thought Japan wanted world domination just like Hitler before he died, #2 too save American Lives, #3 Revenge for pearl Harbor
