The main causes were the rise of fascist regimes in Germany and Italy and a imperialist, militarist regime in Japan. Germany and Japan destabilized Europe, and Asia, respectively through military aggression. Germany's actions led to war with Great Britain and France, and ultimately the Soviet Union and the United States. The United States represented a serious threat to Japanese strategic dominance in the Pacific, and when Japan attacked the US Pacific Fleet anchored at Pearl Harbor in December of 1941, it led to war in the Pacific. Hitler's Germany subsequently declared war on the United States, pulling the US into the European war as well.