World War II, also known as the Second World War, was an international conflict fought between the Axis Powers and Allied Forces. In the timeline of The Man in the High Castle, this war ended with the defeat of the Allied Powers, resulting in the takeover of the United States by Japanese and Nazi forces.
According to both the novel and the series, the main split between the two timelines began with the successful assassination of President Franklin D. Roosevelt in 1933 by Giuseppe Zangaras. Afterward, the United States is subsequently led by John Garner (FDR's vice president) and then by Republican John Bricker. These presidents led to the destabilization of the country and both failed to enact the proper reforms to pull the country from the Great Depression. They continue to maintain an isolationist philosophy during the war and, as a result, they are unable to support the United Kingdom and the Soviet Union's fight against the Greater Nazi Reich or defend themselves from the Japanese Empire in the Pacific.
In the following years, Nazi Germany conquered the Soviet Union and exterminated most of its Slavic peoples, and either invaded or forced the surrender of the United Kingdom. The Japanese Empire then destroyed most of the United States Navy fleet in a decisive battle at Pearl Harbor (much more destructive than in our timeline), and conquered in short order Hawaii, Australia, New Zealand and the rest of the Oceania. Hawaii was briefly retaken by the U.S. in February of 1942. By 1945, the Axis Powers were strong enough to launch a successful invasion of the U.S. from both the Pacific and Atlantic coasts.
The Nazis' then detonated the first atomic bomb (dubbed the Heisenberg Device) on Washington D.C., forcing the United States to surrender by late 1945. Despite this, the Axis Powers were forced to fight a hard and tenacious American Resistance for almost two years until September 18th, 1947, when the Nazi Reich declared that the resistance has finally been put down. This day was marked as Victory in America Day (or VA Day) which officially ended the conflict with an Axis victory.
Sir Winston Leonard Spencer-Churchill was a British statesman and the Prime Minister of the United Kingdom during the war. He was also an officer in the British Army, a non-academic historian, a writer, and an artist. He was assassinated in 1940.
Joseph Vissarionovich Stalin was the leader of the Soviet Union from the mid-1920s until his death in 1949. When the Nazis won the war, Stalin was executed, his death effectively caused the collapse of the Union.
Franklin D. Roosevelt
Franklin Delano Roosevelt never managed to become President of the United States. He was assassinated in 1933 by Giuseppe Zangara, an Italian anarchist who apparently expressed a "hate for all rulers". As a result, the USA never recovered from the Great Depression and was incapable and unwilling to help the Allies during World War II, which led to a German victory in Europe and Africa, and a Japanese victory in Asia and the Pacific.
The TV series offers little information about Roosevelt, although it does mention he was assassinated before taking office. While the TV series doesn't specify the identity of the murderer or when the assassination took place, it's generally assumed to be the same as in the novel. The characters in the TV series occasionally discuss Roosevelt's demise. They privately wonder if the world would be a different place if he had survived his murder.
Adolf Hitler was the leader and dictator of Nazi Germany in World War II. He led the military to conquer many countries most notably France and Poland. In the Man in the High Castle, he also invades the United Kingdom, the Soviet Union, Africa, Canada, the United States and South America. He is shown to be still alive and appears in season one and two. He is subsequently assassinated however by Martin Heusmann's men.
In a parallel timeline, WWII was fought from 1938-1945, with the United States entering the war on the side of the Allied Powers after facing an attack from the Japanese Empire. While successful, the Japanese attack failed to eliminate American aircraft carriers, leaving the US Pacific fleet operational to fight the Japanese advances. Allied troops were involved in an eventual invasion of Europe through Occupied France, and through Italy, once German forces were pushed out of Africa. The Soviet Union successfully resisted German advancements, and began pushing back towards Berlin, leaving the Germans to face three separate fronts, which they were ultimately unable to do. Soviet troops eventually reached Berlin, resulting in capitulation of German forces on 2nd May, 1945.
The Japanese Empire came through the war in a relatively better position, though defeated as well. Despite the loss of numerous battleships in Pearl Harbor, American forces were able to wage a successful Pacific "island hopping" Campaign, pushing Japanese forces back towards their homeland. During the course of the war, American scientists were able to develop a working bomb first, which was then used against the cities of Nagasaki and Hiroshima, forcing Japan to surrender.
Film reel taken in Berlin following V-E Day shows Allied Powers exploding several Nazi insignias signaling the defeat of fascism. These film reels eventually found their way to the American Resistance, and were shown in secret in the hopes that it would boost support for resisting the Japanese Empire and the Greater Nazi Reich which now occupy North America.
WWII officially came to an end on 2nd September, 1945 with the official surrender of the Japanese Empire.