German troops march through occupied Warsaw, Poland, after invading the nation on September 1, 1939, and igniting World War II.
German Chancellor Adolf Hitler speaks to Nazi party officials in 1939, the year of the German blitzkrieg into Poland. Denmark, Luxembourg, the Netherlands, Norway and Belgium soon fell under German control. When France came under occupation less than a year later, Britain was the only remaining Western European nation fighting the Third Reich, and the United States had not yet entered the war.
In Asia, Japanese troops occupy a strategic point on Chusan Island on July 14,1939, during the Sino-Japanese War. Japan signed the Tripartite Pact in 1940, formally allying with Germany and Italy, and by 1942 most of the Asian Pacific Rim had come under its domination.
German soldiers on the Esplanade du Trocadero view the Eiffel Tower. In June 1940, German troops marched into Paris, forcing France to capitulate and establish the pro-Axis Vichy French government.
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British Hawker Hurricanes fly in formation during the Battle of Britain in 1940. The planes were a first line of defense against German bombers attacking England. The battle, fought between July 10 and October 31, 1940, was the first major battle to be won in the air. The Royal Air Force's victory thwarted Hitler's plans for invading Britain.
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Smoke rises behind Tower Bridge during the first mass daylight bombing of London on September 7, 1940.
Italian dictator Benito Mussolini, left, with Hitler, center, and other leading Nazis, visits Germany during the war. Italy and Germany formed an alliance before the outbreak of war, but Italy remained a non-belligerent until June 10, 1940, when it declared war on Britain and France. Fighting spread to Greece and North Africa.
German tanks and infantry attack Soviet positions on the Eastern Front. On June 22, 1941, Germany broke its Non-Aggression Pact with the Soviet Union, launching the bloodiest theater of the war. Though the estimates vary greatly, Russia suffered the most war casualties of any nation in World War II -- as many as 13.8 million military deaths. Estimates of civilian deaths from military action, crimes against humanity, starvation and disease are as high as 9 million.