Per historian A.J.P. Taylor’s The Origins of the Second World War, which was something of a classic when it came out, but has since passed on, due to its somewhat nuanced take.
The first thing that Taylor argues, rather effectively, is that no one was more surprised than Hitler to find himself at war with Britain and France in September 1939. Unlike in 1914, when Germany directly attacked France as a strategic corollary to fighting Russia, German in 1939 dismembered Poland with Soviet assistance only to have Britain and France declare war on it. Taylor makes the strong point that Hitler, while certainly being a wicked man, was not a lunatic or a fool until the hubris of success in 1940 caught up with him. Rather, Hitler aimed for diplomatic, not military success, playing his opponents off each other with patience and skill. Every move he made in the late 1930’s, from the Rhineland to the Anchluss to the dismemberment of Czechoslovakia, was accomplished without a shot being fired in anger, and with the acquiescence or even active support of significant sections of the local populations (Discovering the role that Poland, Hungary, and the Slovaks played in the Czech collapse alone makes this book worth it).
So what changed? Hitler pushed his success too far in Czechoslovakia, souring the goodwill of the British people. For it was Britain that was the key player in the crises of the 30’s. France refused to act without Britain, and so in every crisis the British line became the dominant one. Until the Czech crisis, Britain was chiefly concerned with preventing war, on the grounds that war would be the primary evil, and France was chiefly concerned with restraining Germany in order to maintain her own security.
After the Czech crisis, this polarity reversed. Britain gave Poland a guaruntee in order to prevent Hitler from doing to Poland what he did to Czechoslovakia. But there’s a strong argument that Hitler never intended that; that he was serious about only undoing the last Versailles stricture regarding Danzig and East Prussia, and that he expected the same old game: make diplomatic noise and let the Allies bring him a deal. That’s how the Rhineland, the Anschluss, and the Czech crisis worked.
It never occurred to Hitler that Britain had reached its limit, and feeling betrayed by the seizure of Prague, had no desire to accomodate him any further. At the same time, the desire to avoid war had not left them. Instead, they tried to restrain Hitler while keeping Stalin at arm’s length and threaten a war without really wanting to fight one.
As Taylor has it:
The British were overwhelmed by the difficulties of their position — devising policy for a World Power [The Soviet Union], which wanted to turn its back on Europe and yet had to take the lead in European affairs. They distributed guarantees in eastern Europe, and aspired to build up true military alliances. Yet what they wanted in Europe was peace and peaceful revision at the expense of the states which they had guaranteed. They distrusted both Hitler and Stalin; yet strove for peace with the one and for alliance with the other. It is not surprising that they failed in both aims.
–The Origins of the Second World War, pg. 221
It is important to note that once war came and the restraints were taken off of Hitler, he embarked on Total Conquest without a qualm, and devoured states that had never been involved with the crises of the 30’s, such as Norway and Greece. Neither Taylor nor myself intends this as apologetics for the Third Reich, which was manifestly wicked and deservedly crushed. But it’s always worth pointing out the gap between grand strategy and diplomatic policy. The British were playing against themselves and their interests throughout the 1930’s, and so found themselves forced to declare the war they had never wanted.