What event caused great britain and france to declare war on germany? germany invaded poland. germany seized czechoslovakia. germany seized austria. italy and germany formed the axis powers.

The event that caused Great Britain and France to declare war on Germany was that it invaded Poland.

Explanation:

The Invasion of Poland was carried out, without a formal declaration of war, by Nazi Germany from September 1, 1939. Slovakia also participated from the campaign from day one, again occupying the contested areas occupied by Poland after the Munich Agreement in 1938. The episode marked the beginning of World War II in Europe.  

The attack was carried out on two fronts: by Germany, at dawn on September 1, 1939 (one day after Gleiwitz's "false flag" operation and one week after the signing of the Molotov-Ribbentrop Pact), and by the Union Soviet Union, from September 17th.

One of the objectives of the invasion was to divide Polish territory at the end of the operation; Poland should cease to exist as a country and Poles should be exterminated. On October 6, 1939, Poland was divided and annexed by Germany and the Soviet Union.  

On September 3, in response to hostilities, France and the United Kingdom, followed by the Commonwealth, among others, declared war on Germany.