The entry of the USA into WWII in 1941 helped turn the tide in favor of the Allies in several key ways:
1) The US provided critical economic and material support to Britain and other Allied nations through measures like cash-and-carry and lend-lease programs, helping to reinvigorate their supplies and war efforts.
2) After direct entry into the war following the attack on Pearl Harbor, the US was able to rapidly produce vast military resources that overwhelmed the Axis powers.
3) US participation helped the Allies achieve control of the air and sea, restricting German U-boat operations and allowing for successful large-scale Allied campaigns like the D-Day invasion that ultimately led to victory.