The Treaty of Versailles and the League of Nations policy of 'Collective Security', failed to deter German and Italian aggression in the 1920s and 30s. Italy invaded Abyssinia; Germany marched into the Rhineland, and later annexed Austria. British Prime Minister Chamberlain's policy of Appeasement failed, and Czechoslovakia eventually was seized by Germany. After Hitler invaded Poland, war was declared. Quickly the Germans overran Denmark, Norway, Holland, Belgium, and finally France. Hitler's plan to invade England was thwarted during the Battle of Britain. Children in both Germany and Britain were evacuated from the cities, and the civilian population had to contend with the horror of bombing raids. Hitler turned his attention to Russia, launching Operation Barbarossa. At the same time, the Italians and Germans faced British forces in North Africa. The United States entered the war following the attack on Pearl Harbour, and the German advance into Russia ground to a halt just outside Moscow.