The lead up to WWII began when Hitler took over Austria in 1938. The European powers then appeased him at Munich by also allowing him to take over part of Czechoslovakia. Then he took the rest. Britain and France finally declared war when he invaded Poland in 1939. At first, Germany was successful, conquering France and most of continental Europe, pounding Britain from the air, and even invading its supposed ally, the USSR, in 1941. The U.S. made big profits providing loans, food and arms to the British.
Germany easily conquered France by bypassing its fortifications and using airplanes, tanks and other mobile forces in a new form of coordinated high-speed mobile warfare called blitzkrieg (lightning war). Britain fought hard and narrowly withstood heavy German air attacks in the Battle of Britain, thus avoiding invasion. On the European continent, the Russian-led Soviet Union (USSR) did most of the fighting against Germany. (The Soviet Union accounted for 75% of German deaths and suffered 22 million dead. The U.S. lost 400,000.) After early defeats and heavy losses, the Russians won key battles such as Stalingrad and Kursk, and took over Eastern Europe and part of Germany.
Meanwhile, the militarist Japanese government also invaded its neighbors - Manchuria in 1931, China in 1937 and Southeast Asia in 1940. The U.S. enacted increasingly harsh embargoes on Japan in retaliation. However, only when Japan attacked Pearl Harbor in December 1941 did the U.S. directly enter the war.
In heavy fighting, Britain and Australia turned back the Japanese in Burma and New Guinea. After initial losses, the U.S. defeated Japan in the Pacific in battles such as Midway, Iwo Jima and Okinawa. Dropping the atomic bomb on Hiroshima and Nagasaki finally ended the war.
After Pearl Harbor brought the U.S. into the war in 1941, the U.S. also massively supplied the allies in Europe. The British and Russians learned to enjoy Spam along with millions of tons of other food. The U.S. produced 300,000 airplanes, millions of trucks, thousands of tanks, hundreds of Liberty Ships (Henry Kaiser’s shipyards were finishing a new ship every day!) and countless other weapons and provisions. The U.S. fought in North Africa, Italy and eventually, after the 1944 Normandy invasion, France and Germany.
On the system level, Germany and Japan saw WWII as their attempt to overturn the existing international order previously dominated by Britain, France and the U.S., in order to gain territory and natural resources controlled by those countries. They were also seeking revenge for the perceived wrongs done to them by the dominant international powers after WWI and in the Naval Disarmament Treaties. WWII can also be seen in terms of the hyper-nationalism and personal ambitions of Hitler’s Nazis and the Japanese militarists. Racism was also a factor. During the war, Germany exterminated 6 million Jews and 2 million other civilians in concentration camps and massacred over a million more Jews during military operations. The Japanese also saw themselves as a superior race that should rule Asia. They killed tens of millions of Chinese and other people, forced thousands of Korean, Chinese, Filipino and Indonesian ‘comfort women’ into prostitution for the Japanese Army, and imposed harsh colonialism under the so-called Greater Asia Co-Prosperity Sphere.