Whilst it is true that in March 1939 Britain was still arguably underprepared for war and, of course, the memories of the first world war had not been forgotten, I have difficulty finding reasons why Britain felt prepared to go to war 6 months later, over Poland. Many of the arguments against going to war over the invasion of Moravia, Bohemia & Prague in March do not seem to alter when we analyse the situation in September: geographical positioning was still awkward, public opinion largely had been outraged after the attack on Prague and in terms of the economy/ military issues, surely whatever was achieved in terms of bettering these circumstances would have been less that Czechoslovakia, as a force against the Nazi's, could have provided?
At this late stage [March 1939] surely war was inevitable anyway...
As for the argument that the gov. had not decided about an alliance with Russia in March, I find this unconvincing - originally the rejected Stalin's request entierly and then sent an envoy by slow boat who, since he had no real power, had to refer back to London constantly! - I think Britain had already decided about the Soviet Union [well, Chamberlain had at least, after all she was not invited to Munich etc] so essentially waiting 6 months just meant that they lost the force of Czechoslovakia & the Nazi-Soviet Pact was made, clearly strengtherning Hitler's position.
Could it just have been that there was no respectable way to back out of the Polish agreement? [But that Britain didn't actually expect it to lead to war] whilst there HAD been a way to back out of Munich [since Hitler forced the Slovakians to declare themselves an independent state?] - Britain wanted to retain some amount to status and security, if they had not acted in September, any chance of this would have been lost..?
It seems to me that they DID loose respect/ status/ force of Czechoslovakia and also gave the Nazi's & Soviet's a chance to make their 'non-agression Pact'
March or September... which was more advantageous? [in March was Chamberlain still clinging to the possibility of peace?]