World War II

World War II was a global conflict on planet Earth, begun when the nation-state of Germany, under the leadership of Adolph Hitler and the Nazi Party, invaded Poland in 1939. The Allied nations, including Great Britain declared war on the Axis Powers, including Germany, Italy and Japan. The United States of America joined the war in 1941, when Japanese forces bombed the U.S. naval station at Pearl Harbor, Hawaii. The war ended in an Allied victory in 1945, following the detonation of atomic weapons over two Japanese cities.

Benny Russell served in the U.S. Navy during World War II. (DS9 novel: Unity)

Alternate Timelines
The interference of Doctor Leonard McCoy following his journey through the Guardian of Forever to the year 1930 led to the delay of the United States' entry into the war until 1944. In this timeline, the war continued into 1955. (TOS episode: "The City on the Edge of Forever"; novel: Crucible: McCoy: Provenance of Shadows)

As a part of the Temporal Cold War, the Na'kuhl aided the German war effort, enabling them to occupy the American northeast in 1944. This timeline was negated by the efforts of Captain Jonathan Archer. (ENT episodes: "Storm Front, Parts I and II")