The discussion whether or not the western allies "betrayed" Poland is pointless. They had signed the alliance (not only with Poland but also with the CSR) in a time when they were still believing the world worked the way it did in 1914. The never realized that this was no longer the case until it was too late... probably until Hitler occupied Prague in March 1939.
In the west, people believed for a long time that wars like the Great War would simply not happen again. They thought, hey, we've suffered so much for so little, surely the Germans think the same. They didn't realize what Hitler's intentions were. Hitler was extremely good at hiding his true thoughts when it was opportune... remember how he always talked about "bringing all Germans home into the Reich"? That's what he said until after Munich, and everyone wanted to believe it. They didn't want to believe that he would want to conquer Poland, or that he would deliberately attack France and England. It's the common (flawed) logic - "It must not be, so it can not be."
Hitler reoccupied the Rhineland - hey, he broke the Versailles treaty, but at the time many people in France were already believing the treaty was unjust in the first place.
He annexed Austria - and newsreels and newspapers all showed how the overwhelming majority of the Austrians cheered the invaders, how they stood at the roadsides and waved swastika flags. It's like the TV pictures today of cheering Iraqi kids - you see those pictures and you think "But it's all good!" and you forget that international law was broken, that you were actually against it. The Nazi propagandists really knew what they were doing.
Hitler then asked for the last thing he would demand - the last thing, then he would be satisfied. He wanted the German-populated border regions incorporated into the Reich, and then there would be no more quarrels between the CSR and Germany. Only this item was still standing between the two nations, only this was still preventing "peace in our time". And the people in the west, they saw that they were selling their own ally, that they were serving the CSR on a silver tablet to Hitler, but they wanted to believe that peace was possible. They wanted to believe that this was all Hitler wanted. Surely Hitler would not want war! Why, he always spoke of how seriously he was committed to peace, that he wanted merely a just peace betwen equals. And at that point, France and Britain were no longer able to negotiate from a position of strength. They had been deluding themselves for too long... it was too late to turn the tables, to stand up to Hitler and say "no!"... or so they thought. The world had seen that this Germany was a disciplined country, that Hitler had set things to right. When people thought of Germany those days, they thought of a country that had mysteriously been transformed within only a few years, from a down-trodden, demoralized country suffering from the great depression into a powerhouse - led by a man who had no doubts about what he wanted, a man driven by an iron determination and fueled by a desire for peace. Or so everyone thought. To stand up to this man would not only be asking to be kicked in the ass, it would also be wrong. Hitler was on the Times cover as "Man of the Year 1938". He was the class champion, and people looked at him the same way they would look at the football quarterback in their class who aced all his exams and also played on the soccer varsity team and had the best-looking girlfreind in the entire school. One of those people who had "SUCCESS" stamped on their forehead.
And Hitler played along with their game. (He had actually wanted all of Chekhoslovakia, not just the Sudetenland.) People wept as Chamberlain proclaimed that there would be peace in our time. The west was psychologically just not able to stand up to Hitler - until March 1939, that's when people were finally able to see the what Hitler and the Nazis were really up to.
In a brilliant display of brutal power politics, Hitler forced the Chekh president Hasha to agree to the German occupation and the independence of Slovakia. This is when everyone who was still under Hitler's spell realized that the Munich treaty was not worth the paper it was printed on. But it was also too late... Britain and France forged an alliance with Poland, Romania and Greece, but it was a makeshift agreement, and personally I doubt the statesmen who signed the treaty believed that this would intimidate Hitler.
The great dilemma on September 1st was that France and Great Britain, the allies of Poland, had not concerned themselves to the question "How can Hitler be stopped" until it was too late, but that they had nonetheless bound themselves to Poland. They had bound themselves although they had no idea how to fulfill their promise. They declared war on Germany, alright, but Hitler was determined to go to war with them anyways, so that did not matter. Had they remained neutral, he would have come for them right after he would be done with Poland.
They had armies, alright, but they had never thought about using them until it was too late. Armies are trained to fight the previous war, not the upcoming one... The French had not contemplated that they would declare war and the enemy would not come. They called the month until May 1940 the "Phoney War" with a reason. It was a war that they had imagined to be totally different...
Macchiavelli would say that they should never have signed the treaty in the first place. Never promise to do something you have no idea how to do. He also had a few very clear things to say about military readiness... *sigh*
