On Sept. 1, 1939, Germany invaded Poland under the pretense of having been attacked by the Poles (it was staged by Nazis). This is generally regarded as being the beginning of World War II. Tens of millions of people would be killed in Europe and Asia before the end of the war.
Here is the British poet W.H. Auden’s poem “September 1, 1939” about the break-out of the war.