> Did Europe intervene in the American Civil War,

Thankfully not, as the upper class in Britain at least tended to favour the Confederacy.

> East Asia

There's few parts of the world you could list that is a worse example of places Europe haven't intervened.

European powers were either one of the major sides or a supporting power in most wars in East Asia for a substantial part of the last couple of centuries.

You do realize the US "inherited" the Vietnam war with the associated campaigns in Laos and Cambodia from France, do you not?

That the US fought with the British, French and Russian empires against China in the Opium wars?

Now, arguably those are not good arguments for continued intervention. On the contrary. While I think there were some very good reasons why the US ought to have intervened in World War II earlier, I also wish the US had learned more lessons from some of the wars mentioned above (and a lot of other foolish wars the European colonial powers were involved in).

Instead the US seems to have seen the way the waning colonial powers acted as a recipe for how it needed to behave as its own power grew, with - for anyone paying attention to the outcomes of the colonial wars - predictably horrible outcomes.

> Even the rest of Europe abandoned Austria and Czechoslovakia to the Germans before Poland fell

It's easy to look back and regret those. But the politics around them, and the patchy history of Europe, made them politically "tricky". The Austrian "Anschluss" happened on the background of the German Austria's previous attempts to merge with Germany, which was denied it by treaty, including the Treaty of Versailles. In 1938 Austria was under a semi-fascist dictatorship, and there was then pressure for a referendum on the issue (with extensive support and threats from Hitler). Facing lack of support from Mussolini who had previously protected them against Hitler, the Austrian government accepted a large number of nazis into government, who then used their positions to prepare and conduct a coup against the austro-fascists supported by threats of force on the basis of allegations of plans to rig the referendum. It was then the Austrian nazi government that handed control over to Germany in a manner designed to effectively be a request from the Austrian state for the Germans to enter Austria. They then carried out a vote to legitimise the takeover.

The combination of events made it politically extremely hard for outsiders to do much more than complain. Europe should go to war because a government asked for military support from a neighbour following allegations that the previous dictator may have tried to prevent the will of the people, in the face of widespread public support for the nazis? In retrospect, probably. But without the knowledge of what Hitler went on to do, it was squabbling between two authoritarian governments over control of an artificially created country with a history of wanting to merge, where the new dictator likely had as much or maybe even more support than the old dictator.

In the case of Czechoslovakia similarly, the state was an artifical creation from parts of the Austro-Hungarian Empire, that included a substantially ethnically German population in certain areas (Germans made up 23% of the population of Czechoslovakia) that had genuine grievances of oppression against the elite of the country, and the government also purposefully moved parts of the German population to the West, making it even easier to make excuses.

So Hitler was similarly able to play the role of someone who at least plausibly wanted to "liberate" the German population, and present the whole thing as "rectifying" a situation that was only 20 years old and an outcome of a previous war when the will of the people in question had been largely ignored when the borders were drawn up, and so got Sudetenland. When Hitler went into the rest of Czechoslovakia, he got the support of Slovakia, and both Hungary and Poland used the opportunity to annex area - he was not the only agressor, and support for going to war over Czechoslovakia when even the government over a substantial part of the twenty year old country took part in the aggression against the rest and supported the German invasion was not an appealing proposition.

While there were certainly fears and lots of people who had concerns about it, Hitler played people up against each other, and was dealing with governments who all had gotten their hands dirty in drawing up fairly arbitrary borders without the involvement of the affected people and/or through wars within the last few decades. Combine that with a strong degree of denial - nobody wanted to rush into a new war - and a lot of admiration for what Hitler had achieved domestically in Germany, coupled with a grudging acceptance that perhaps giving these people what they seemed to want might not be such a bad idea if it resolved some of the remaining tensions created by the Treaty of Versailles.

In retrospect, of course, that was stupid. But then in retrospect it was stupid to not intervene when Hitler remilitarised areas Germany was not allowed to militarise and in a whole host of other situations leading up to the war.

But the cases of Czechoslovakia and Austria were still substantially different than the case of Poland, both because they came first, but also because they were clouded in politics and could be "explained away" as something other than naked aggression. By the time of the invasion of Poland, though, there was no denying that Hitlers ambitions did not stop at "liberating" German speaking groups in weird little newly created states, and with the rapid fall of further states