In 1939, Britain and France declared war on Germany after it invaded which country?

In 1939, Britain and France declared war on Germany after it invaded Poland. This action was a direct response to Germany's aggression and violation of international treaties. The invasion of Poland by Germany was seen as a clear act of aggression, violating the sovereignty and territorial integrity of another nation. Britain and France, recognizing the need to defend the principles of international law and prevent further aggression, chose to declare war on Germany to signal their opposition and support Poland against the German invasion.