Invasions of people--many of the groups came as military and fought the Romans as invading forces.
Counterclaim--they were migration patterns as they were attempting to gain access to an empire with advanced trade and education. Migrating to the Roman Empire gave a person or group with extra resources. 
        
                    
             
        
        
        
The Japanese bombed Pearl Harbor before America joined the war. America did not plan on joining the war, but was going to support their allies with resources. Japan had wanted exports from America, but the U.S. refused. Since the American government would not give to Japan, Japan decided to attack Pearl Harbor. This made the Americans furious as they had not yet enlisted and saw this as an act of terrorism. They immediately took action and joined the war.<span />