Simplify the following text for a Grade 7 native English-speaking person.
Original: The war is generally accepted to have begun on 1 September 1939 , with the invasion of Poland by Germany and Slovakia , and subsequent declarations of war on Germany by France and most of the countries of the British Empire and Commonwealth .
Simplified: The start of the war is generally said to be on September 1 , 1939 , with the invasion of Poland .