World War II was a global conflict lasting from 1939 to 1945 opposing the Allied and the Axis Powers. It ended with the Allies beating the Axis Powers. It's the greatest war fought in the whole history.
When Germany invaded Poland on September 1st, 1939, together with the Soviet Union, France and England promptly declared war on the Third Reich. After that, in 1940, Denmark, Norway, Belgium, the Netherlands and France were invaded and a part of oriental Europe assimilated. France was taken in five weeks due to a evasion by the Nazi of the Margenau Line, through the forests of Belgium. In 1943, Germany lost the war in Stalingrad, where the 6th Army was completely destroyed, and then on Kursk, the greatest armored battle in history. After that, the Soviet Union's advance was unstoppable.
Several battles happened but the most memorable of them were: Tobruk and El Alamein (both in north Africa - the German forces under the command of Erwin Rommel against Britain), Stalingrad and Kursk (Germany met Soviet Union in both of them), Midway (Japan facing the U.S.), and Iwo Jima (U.S. vs. Japan).
However, in the series, World War II was just a big fight between the children of Poseidon and Zeus on one side and the children of Hades on the other. Hades states that his children were on the losing side, which means all of the children of Zeus and Poseidon were the Allies and the children of Hades were the Axis powers. The war ended after the nuclear attacks on Hiroshima and Nagasaki.
Later, it's discovered Adolf Hitler was the Roman demigod son of Pluto, who fought against Franklin Delano Roosevelt, son of Zeus, Winston Churchill, son of Poseidon, and Joseph Stalin, son of Hades (during the second half of the war).
This led to the pact of the Big Three because their children were affecting the course of history too much. It was decided more demigod children would be dangerous for the mortal world.
Frederick Chase is also very knowledgeable on the subject, being a professor on war.
- Curiously, another god was shown fighting against Egyptian Demons in Nazi uniforms during this time. It is unclear what role this played in WWII.