when did the us enter ww

The United States entered World War II after the Japanese attack on Pearl Harbor on December 7, 1941, and formally declared war on Japan on December 8, 1941. A few days later, on December 11, 1941, Germany and Italy declared war on the United States, and the U.S. responded by declaring war on them the same day.

Because your query says “when did the us enter ww” (without the number), here are both world wars for clarity:

• World War I: The United States entered World War I when Congress declared war on Germany on April 6, 1917, ending its earlier neutrality.

• World War II: The United States entered World War II in early December 1941, immediately after the Pearl Harbor attack, through a declaration of war on Japan on December 8, followed by declarations against Germany and Italy on December 11.

TL;DR:

• WWI entry: April 6, 1917
• WWII entry: December 7–8, 1941 (Pearl Harbor attack and U.S. declaration of war on Japan)

Information gathered from public forums or data available on the internet and portrayed here.