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

Quick Scoop: When did WW2 start?

Core date

  • The most widely accepted start date of World War II is 1 September 1939.
  • On that day, Germany, led by Adolf Hitler, launched a full-scale invasion of Poland without a formal declaration of war.

How it turned into a “world” war

  • In response to the invasion of Poland, Britain and France declared war on Germany on 3 September 1939 , turning a regional conflict into a wider European war.
  • Within days, the British and French empires (except Ireland) were drawn in, which helped transform the conflict into a truly global war over time.

Why some people give different “start” dates

Historians sometimes point to other earlier events as the roots of WW2:

  • Japanese invasion of Manchuria (1931) – seen as an early step in Japanese expansion in Asia.
  • Second Sino‑Japanese War (from 7 July 1937) – many scholars consider this the start of the Pacific side of the conflict.

However, despite these earlier conflicts, 1 September 1939 remains the standard answer to “when did WW2 start?” in most history books and school curricula.

TL;DR: World War II started on 1 September 1939, when Germany invaded Poland; Britain and France declared war on Germany on 3 September 1939, rapidly escalating it into a European and then global war.

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