World War II was the global military conflict that took place between 1939 and 1945. It was the largest and deadliest war in history.
The war was waged between the Axis powers, chiefly Nazi Germany (whose invasion to Poland spurred the conflict), Fascist Italy and Japan, and the Allies, mainly the United States, Great Britain, and the Soviet Union. Originally, Germany surrendered in May 1945 and Japan surrendered in September 1945.
Vandal Savage believed that World War II was his best opportunity to achieve world domination. In the early 21st century, he sent historical and technological information to his younger self in order to enable a takeover of Nazi Germany, and with his knowledge of future events and a half-century head start on technology, to win the war and achieve global domination.
Savage was initially successful. He took control of Nazi Germany and had Adolf Hitler cryogenically frozen for reasons unknown. Under Vandal Savage, the Nazis thwarted the Allied invasion of Normandy and turned the tide of the war in Europe in their favor. With their advanced weaponry, such as War Wheels and jet engines combined with foreknowledge of their enemies' capabilities and movements, the Nazis closed an iron fist around Europe and invaded the United States of America. This gave Vandal Savage the world domination he had been seeking, and his regime ruled into the early 21st century.
Fortunately, chance protected the majority of the Justice League from the changes to the timeline, and the League, after discovering what Savage had done, was able to travel back in time as well. They arrived in Europe at a point in time after June 1944 as evidenced by the presence of American military forces in Europe. By this time, Savage had already managed to produce futuristic weaponry and equipment for the Nazis.
The Justice League members worked with the Blackhawks, Easy Company, Steve Trevor and the United States Armed Forces to thwart Savage's plans. With the League's assistance, the Allies were able to stop the invasion of America, destroy the Nazis' information about the future and new technologies and send Vandal Savage to the bottom of the Atlantic Ocean.
After Savage was defeated, the Nazis awakened Adolf Hitler, to resume his position as Führer. The extent of the damage to the timeline is unknown, but is assumed to be minor as no major changes were apparent after Savage's defeat and the Justice League's return to their proper time.