The United States passed the Lease Lend Act in 1941, which allowed the U.S. to supply allied nations and signed the Atlantic Charter with Great Britain, which stated their joint opposition to fascism in August 1941. Though the United States technically remained neutral, these were two significant steps towards entering the war.
The Soviet Union switched sides from the Axis to the Allies after Germany invaded the Soviet Union in June, 1941. In October, the Germans reached their deepest penetration of Soviet territory when they got within 60 miles of Moscow. However, the cold temperatures and staunch defense forced Germany to give up on its attack on December 5, 1941.
In March, the Nazi concentration camps became operational. The plan for exterminating the Jews, known as the Final Solution, was signed in July, 1941. Over 6 million Jews and 3 million others would be killed before the end of the war.
In 1941, the war became truly global when the United States entered the war after the bombing of Pearl Harbor by the Japanese on December 7, 1941.
When Germany attacked the Soviet Union, it was forced to fight a two-front war meaning it had to split its troops between fighting Great Britain in the west and the Soviets in the east. Similarly, Japan opened a two-front war when it attacked the United States because it was also fighting enemies in Asia.