They declared war and did nothing (hence names like Sitzkrieg or Phoney War). Since Polish defense strategy was based on assumption that UK and France will fulfill their obligations it was a much easier win for Germany than it could've been otherwise. I'm suprised it's not talked about more outside Poland, but this betrayal cost millions of lives.

What betrayal? The UK and France did go to war against Germany, but they were not in a position to invade Germany in 1939. France suffered more than 4 years of German occupation as a result of its decision to declare war on Germany, a pretty high price for honoring its obligations to Poland.

France did not honor it's obligations and if it wasn't in position to attack Germany then France shouldn't have made those guarantees (Maurice Gamelin's "bold relief offensive"). Thinking that Germany wouldn't have attacked France if war hadn't been declared is a bit naive.