Answer:
**United States:**
1. **Attack on Pearl Harbor (1941):** The major catalyst for the U.S. entry into World War II was the surprise attack by the Japanese on the U.S. Pacific Fleet at Pearl Harbor on December 7, 1941. This event led to the United States declaring war on Japan the following day.
2. **Axis Declarations of War:** Germany and Italy, part of the Axis powers, declared war on the United States on December 11, 1941. This declaration, in response to the U.S. entering the war against Japan, further solidified the United States' involvement in the broader conflict.
**Soviet Union:**
1. **German Invasion (1941):** The Soviet Union entered World War II following the German invasion on June 22, 1941, in a massive military operation known as Operation Barbarossa. This invasion violated the Molotov-Ribbentrop Pact between Nazi Germany and the Soviet Union, leading to the Soviets joining the Allies in their fight against the Axis powers.
2. **Siege of Leningrad and Battle of Moscow:** The brutal Siege of Leningrad and the determined defense in the Battle of Moscow were critical early events that showcased the Soviet Union's resilience against the German advance. These battles marked a turning point, signaling the Soviet determination to resist and ultimately defeat the German forces.
These events collectively brought both the United States and the Soviet Union into active participation in World War II on the side of the Allies against the Axis powers.
