When Nazi Germany invaded Poland the allies, The United Kingdom and France, promptly declared war on Germany and did....nothing. Well, not exactly nothing, they did manage a half hearted offensive the only succeeded in moving French forces a few miles into German territory and get 2,000 French soldiers killed. The British did nothing in terms of major military movements into German territory, and outside of some air combat, did little against the Germans. The first major ground fighting between the United Kingdom and Germany didn't occur until the Norwegian campaign.
All in all the inactivity by the Allied nations was pretty damning considering that only 23 German divisions were left to face almost 110 divisions of the Allies. Even with the military incompetence of the French generals who, despite the fact that the implementation of armored vehicles in World War I lead to their victory, failed to implement a combat strategy that effectively utilized armored units and adhered to World War I military doctrine. The Allied nations left Poland to be ravished by the Germans, and later the Soviets, when decisive action early on could have ended the war quickly. This inaction would have tremendous consequences for decades to come as both the Allied power inaction, and the United States ignoring of the possibility of a Japanese attack, has been one of the justifications for a very aggressive foreign policy since then. Millions of deaths could have been prevented if the United Kingdom and France had only acted more aggressively.
The phrase 'hindsight is 20/20' is often used as a response to could've, should've and would've situations. But the fact is that the allied nations inactivity was utterly inexplicable. Nazi Germany was very open about it's desires to establish a Greater Germany Reich, which would have included taking territory from the French, and the bloodless invasion of Czechoslovakia should have made it abundantly clear that Nazi Germany was testing the waters. When the Germans invaded Poland there should have been no doubt between the allied nations, they were next on the agenda. It's one thing to dismiss something as bellicose rhetoric or saber rattling when a nation doesn't escalate, like North Korea, but it is entirely another matter when a nation unambiguously push the boundaries.
While Neville Chamberlain's appeasement of Germany when it came to Czechoslovakia could be seen as a betrayal, and a gross miscalculation, it is somewhat understandable. Britain had only recently fought a bloody, and to many British largely pointless, war and sought to avoid another bloody conflict that would exhaust an already weakened empire. I understand the logic, though based on a faulty premise, behind appeasement on Czechoslovakia. But once Germany had exposed its willingness to use force to achieve its ends, they should have acted decisively. The fact that they didn't was a terrible travesty with consequences that are still reverberating today.