What event brought Britain and France into the war?

Answer:
The main major event that got Britain and France involved in WWII was the invasion of Poland by the Nazis. The three countries, Britain, France, and Poland had an agreement that if any one of them was invaded the other two would attack Germany.
First answer by JadeIsNotEdible. Last edit by JadeIsNotEdible. Contributor trust: 13 [recommend contributor recommended]. Question popularity: 1 [recommend question].