Question

why was Second World War happened

Answer 1

It involved many of the world'scountries. The Second World War wasstarted by Germany in an unprovoked attack on Poland. Britain and France declared war on Germany after Hitler had refused to abort his invasion of Poland.
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Answer 2

The two dates most often mentioned as “the beginning of World War II” are July 7, 1937, when the “Marco Polo Bridge Incident” led to a prolonged war between Japan and China, and September 1, 1939, when Germany invaded Poland, which led Britain and France to declare war on Hitler's Nazi state in retaliation.