What actions explains why Britain officially enter World War II

Hitlers invasion of Poland.

Explanation:

Before the invasion of Poland, France and the UK tried to appease Hitler bu giving him everything he wanted. They drew the line at an invasion of Poland though.

Britain and France declare war on Germany. On September 3, 1939, in response to Hitler's invasion of Poland, Britain and France, both allies of the overrun nation declare war on Germany ... They were also working under orders not to harm German civilians. The German military, of course, had no such restrictions.