When did great Britain and France declare war against Germany why?

Why:

- Invasion of Poland: Germany invaded Poland on September 1, 1939, violating the Anglo-Polish military alliance.

- Failure of diplomatic efforts: Britain and France had attempted to negotiate with Germany to resolve the crisis, but these efforts failed.

- Desire to prevent a wider European war: Britain and France believed that Germany's aggression would lead to a wider conflict if left unchecked.