Why did Britain and france declare war on Germany for invading Poland during the world war II?

In mid- 1939 Britain and France were caught pants down by the german military build-up - mostly because of the activities of "peaceniks" and soviet agents (the soviets and nazis were allies at that time). They urgently needed someone to make a "second front" and take pressure away from them- so they supported Poland (which had multiple territorial disputes with germany) as their ally. And signed the necessary military agreement to make the deal official.

When push came to shove both Britain and France did nothing except political declarations. they sent no troops, no planes no ships to safeguard Poland. just words. They did not even declare war on the soviets- who also attacked Poland in 1939. And who still have not returned the land they stole as hitler's allies. not to mention the milions of murdered Polish citizens

There had been expansion of the German empire (The Third Reich) in the 1930s - Chezcoslovakia, Sudetenland, Austria were all annexed without much opposition - due to the other powers seeking to appease Hitler. This threatened the stability of Europe (especially as both Spain and Italy were Facist controlled). Poland was seen as the point beyond which Hitler could not go.

After Germany signed a pact with Russia, Britain and France did the same with Poland - which was clearly under threat because it divided Germany into two - part of the rather badly ordered settlement of WW1.

Had the Soviet Union not been ideologically opposed to Germany there might have been more cause for concern about their pact. Part of Churchill's skill was rehabilitating 'Uncle Joe' as an ally when Hitler turned on Stalin in 1941.

Britain and France had a treaty with Poland to guarantee its defense in case it was attacked by another nation. I find it very interesting that even though Germany invaded Poland, so did Russia (It was an ally of Germany at the time) and yet Britain and France did not declare war on Russia.

A conflict from 1939-1945 which grew to become into fought by skill of each and all the international. interior the top, the Axis powers, that are Italy, Germany, Japan and a few greater international locations lost and so did hundreds of thousands of lives.