What are the causes for human migration and the refugee crisis in the modern era?

nativists tried to get rid of the immigrants as they thought they were going to ruin the u.s.  

roosevelt believed that in light of the country's recent military successes, it was unnecessary to use force to achieve foreign policy goals, so long as the military could threaten force. this belief, and his strategy of “speaking softly and carrying a big stick,” shaped much of roosevelt's foreign policy.