It really is as simple as appeasement. The British did not want to engage the Germans in war, and leading up to that Chamberlain did not want to believe the rise of Hitler was going to lead to another World War. The appeasement, from the perspective of the British, was to avoid another war. This was a common sentiment among British intellectuals as well, John Maynard Keynes famously said that the Treaty of Versailles would lead to another World War. The harshness of the treaty itself is debatable, but the appeasement and it's consequences are really not. The lax policy of Britain is the exact reason why Germany was able to break and violate the treaty and take Czechoslovakia. France was much more militant about upholding the treaty, they wanted a more severe treaty in the first place, but faced public scrutiny when they attempted to uphold it. For example: the French sent troops into the Rhur in 1923 to take coal from the Germans as reparations, and they were met with public scorn and a lack of support from their allies. This gave the French a bit of a PR black eye and explains why when the Germans violated the treaty multiple times, nobody really stopped them. Poland itself was such a serious overstepping and a declaration of war that the allies could no longer ignore it.