World War II is generally considered to have started on 1 September 1939, when Nazi Germany invaded Poland.

Core date

  • The most widely accepted start date is 1 September 1939, marked by Germany’s invasion of Poland.
  • Britain and France responded by declaring war on Germany on 3 September 1939, turning the crisis into a broader European war.

Why that date matters

  • The invasion of Poland was a deliberate expansionist move by Hitler, breaking previous agreements and shocking other European powers.
  • These declarations of war by Britain and France are why most historians treat early September 1939 as the true beginning of World War II as a global conflict in the making.

Other “starting points” people mention

  • Some discussions focus on the Pacific and point to earlier Japanese aggression, such as the 1937 outbreak of the Second Sino‑Japanese War or even the 1931 invasion of Manchuria.
  • Others casually say “1941” because the United States entered the war after the attack on Pearl Harbor in December 1941, but this reflects a U.S.-centered view rather than the actual start of WWII.

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