What was the source of the conflict between the united states and mexico before they went to war?

It was caused by a border dispute.  Mexico never recognized Texas independence.  They claimed that the Rio Grande was still part of Mexico.  This further aggravated when President Polk sent troops to the Nueces River as the expedition to California by John Fremont.  

missouri compromise of 1820 which prohibited slavery

roosevelt essentially took the opposite approach as hoover in taking on the great depression. herbert hoover thought that america and its economy could naturally recover from the depression in due course, so he adamantly restrained the federal government from intervening on behalf of the people affected. on the other hand, roosevelt dramatically increased employment by expanding the federal government and establishing agencies that would aid in relieving some of the country's worst problems.