Actually, Nazi Germany and perhaps the Soviet Union were the only states that presented a direct and existential threat to the UK, France, and to an extent, the whole of Europe at the time.
Wrong. Imperial Japan was expanding in the far-East and threatened British colonies like Signapore and the Raj (jewel in the crown of the British Empire). French colonies in Indochina were also threatened. Then there was Italy, which had moved into Ethiopia and and had made moves into Southern Europe, which put it in a position to attack the Allies in the Mediterranean and North Africa.
And in regards to the Soviet Union, there is no 'perhaps' about it. The Soviet Union wielded a far larger military force than Nazi Germany, and its ideology of Communism was on a direct collision course with West. When Germany first started expanding, it really didn't seem like a greater threat. Propaganda efforts meant that a good many people admired Hitler, and the demands he made weren't all that outrageous, at first.
They chose to ignore/appease what was by far the largest and the most imminent threat for them at the time.
Again, the Soviet Union seemed much more dangerous at the time. Considerably larger army, more toxic ideology and speeding towards industrialisation.
Reasons? Like what?
Like having a bulwark against expansionist communism? Like preventing a war that would have given an open goal to other adversaries? Like buying time to put their countries on a war footing?
The British desire to desperately hold on to its failing empire?
Well, yes. But they didn't know it was failing at the time. And keep in mind that the Empire gave Britain access to valuable resources and gave them a good deal of strategic advantages.
And you are, again, criticising with the benefit of hindsight.
Keeping peace with Hitler would only delay the inevitable and everyone with a functioning brain knew it, even back then.
Uh no, they didn't. Many people wanted to try diplomacy before rushing into bloodshed. The memory of the carnage of WW1 was still burned in many people's minds.
But they had to have known Hitler would come for them before he came for Russia. It was a simple matter of strategy, geography and logistics.
No, it wasn't. Hitler's primary target was Russia and Communism, the ideology which was diametrically opposed to Nazism. And I'm not sure why you see Hitler moving against the West as a matter of 'strategy, geography and logistics". The Ardennes Forest was seen as impassable for German tanks, and the Maginot Line was seen to be impenetrable. Of course, Germany did end up defeating France rather quickly, but that was a remarkable fluke and one that could not have possibly been predicted.
So again, you are unfairly judging British/French policy with the benefit of hindsight. It was hard to envisage how Germany could break through French lines so quickly, and Hitler's long-standing hatred of Communism made it unlikely that he would attack the West first. and expose his Eastern border to that threat.
And look how well it all turned out for them. All of their fears came to pass because they adopted the Jeremy Corbyn approach.
Was there really any choice? I'm not seeing an opportunity when France/UK could have toppled Hitler. The Rhineland affair was too early and they simply weren't prepared. If France/UK made it clear they wanted to totally defeat and humiliate Germany, and depose Hitler with force, don't think there wouldn't have been bitter, bitter resistance. Every opportunity before 1939 seemed like too big a risk to take. The fact that they actually tried diplomacy, which actually bought us time to rearm and fight the War more effectively when it came, really isn't all that outrageous.
They could have nipped Hitler in the bud and prevented a greater disaster.
When, exactly? They could have confronted Germany early on in the Rhineland, but it would be very unlikely that France would have launched an effort to actually depose Hitler at that stage. They weren't on a war footing, and the public support simply wasn't there at all. Therefore, a confrontation at that point wouldn't have changed course all that much. Then there was the Austrian Anschluss, which likely did have support from the Austrian people and was permitted by Italy (now an ally of Germany, which would have made a war against Germany even more of a risk). Then later, there was the incident against Czechoslovakia, but many saw Czechoslovakia as rather distant and the Treaty of Munich as a decent compromise. And again, France and the UK had to weigh up the other threats waiting to strike at them across the globe, and the fact that they had not completed rearmament at that stage.
And don't tell me that hindsight is 20/20 because even at the time people, people like Winston goddamn Churchill, knew that shit was inevitably going to hit the fan. To quote the man himself: "You were given the choice between war and dishonor. You chose dishonor, and you will have war.”
Yep, and he was seen as a bit of lunatic. We can say now confidently that Britain and France should have probably woke up and smelt the coffee. But back then, the idea that France/UK should strike at Germany was a serious dilemma, for a variety of reasons that have already been laid out.