What caused great britain and france to declare war on germany in 1939?

In 1939 Hitler was becoming more and more aggressive with his foreign policies and decided to invade Czechoslovakia. The entire European continent was worry that war could soon be in their door steps.

The British and French fearing the worst of Germany´s actions agreed to support Poland in case that Germany decided to also invade them.  

Unfortunately, this was not of any concern to Hitler, and his Nazi Germany continue with the aggression and on September 1st 1939,  attacked Poland which was overwhelmed by the Nazi war machine.

After this war like action, the British and French had no more options but give Hitler an ultimatum, to withdraw his forces from the newly occupied Poland. this of course have not effect on Germany and both British and French officially declared war on Hitler and his Nazi Germany.  

yes

explanation:

why