The German invasion of Poland two days earlier triggered the French and British governments’ guarantee to Poland, so after the expiry of the ultimatum, war was declared. Here’s British PM Neville Chamberlain’s announcement of the state of war (RAM file, not WMV).
Unlike the situation in 1914, where Canada was automatically at war when Britain went to war, in 1939 Canada was able to decide whether and when to go to war. It wasn’t until a week later, on September 10, that Canada was formally at war with Germany.
