The direct answer to this question is as follows: A. The German invasion of Poland on Sunday, September 1, 1939. B. By September 3, 1939 Britain and France held to their treaty to protect Poland and war was declared on Germany.
Germany fully expected this and already had created a strategy for the invasion of France.
These are the 2 things that began WW2. The circumstances that led up to WW2 are quite extensive and not pertinent to the question asked.
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The Germans started both wars.
The Great War was, at the time, unique. When the next war started, the names World War One and World War Two were used because World War One was no longer a one-of-a-kind huge multinational war.
World War Two started in 1939 when Adolf Hitler began taking over countries. World War Two ended in 1945 after the United States dropped two atomic bombs on Hiroshima and Nagasaki, in Japan. Interestingly, there wasn't an "official" end to World War Two until 1990, when a certain document was signed that officially concluded the hostilities between the countries that fought in World War Two. This is because when Japan unconditionally surrendered, "ending" the war, no treaty was signed that officially ended the war. The countries simply stopped fighting.
World War 1 started in 1914 and ended in 1918. World War II, was the amalgamation of what had initially been two separate conflicts. The first began in Asia in 1937 as the Second Sino-Japanese War; the other began in Europe in 1939 with the German invasion of Poland, however World War 2 officially started on September 1, 1939.
World War I ended with the Treaty of Versailles on November 11, 1918. World War II is generally considered to have started on 1 September 1939. So a bit (two months and ten days) under 21 years.