Chamberlain tried to do a deal with Hitler before he invaded Poland. When they finally invaded on Sept 1st 1939 it was clear no deal would work and the Brits declared war on Hitler on Sept 2nd 1939. We didn't declare war on him for another 2 years, so i don't think we've got any right to be smug about Chamberlains mistake.
No doubt. If we'd have been willing to back them up Hitler never would have been free to launch his war.