Why does it matter whether another nation is a democracy? The only reason we have "interests" in Europe and Asia is because the rich capitalist class keeps dragging us over there.
Now you're just spouting off like some dipshit socialist. It's not "the rich" who drive this, it's the fact that the U.S., as a whole, has an interest in making sure no single power dominates Europe and Asia, because that would pose a security threat to the U.S. Perhaps less direct today, but it's a situation that would not serve American interests. Not just those of some class of oligarchs who don't exert the kind of power you seem to think.
As to whether a country is a democracy or not? It doesn't have to be to serve American interests, and I think you're misreading my reasons for backing Taiwan over mainland China. One is a democratic country that is overall quite friendly to U.S. interests (and is the hub of all semiconductor production, without which the rest of the world would be screwed if the PRC took it over). The other is a genocidal totalitarian state bent on trying to bully the rest of the Pacific Rim countries, which is problematic for the U.S. because of longstanding ties and for a variety of reasons.
Also no we haven't tried isolationism and it failed. It kept getting sabotaged by people like you who want the US to be involved in other nations business.
The 1920s and 1930s say hi and would like to point out you're full of it. Also 1945-1950, when it became clear that the Soviet Union wasn't interested in peaceful coexistence with the West despite American attempts otherwise. Including a massive disarmament post war that ended up biting us in the ass when the Korean War broke out.
And notably I'm not saying "we need to be involved everywhere", I'm saying "We have interests in various parts of the world, and sticking our heads in the sand doesn't work."
Like world war 1 was not a failure of isolationism. If the U.S. did not join and Germany won, who cares? They would be stuck in Europe they would not be able to come here to the New World. Same with every other war.
The question isn't whether they could invade, the question is whether they could drive us into the ground economically and basically force a collapse or at least severe weakening of the U.S. economy, which would in turn lead to a massive political crisis the likes of which would make the Great Depression look like a minor hiccup.
So what if China is the master of Asia? Why should America be involved in Asia, America should stay in America? The same for the world wars why should America care if Germany is master of Europe, or later Russia becomes master of Europe? Why should someone in the New World care what happens in the old?
Because as I noted above, there are complex factors that can result in threats to the U.S. that don't involve being invaded, something you really don't seem to get. So let me try to put it this way:
1) The distance from New York to London is about 3,500 miles. That's only a few hundred miles more than the distance to LA, and about a thousand miles closer than Alaska or Hawaii. So, they aren't anywhere near as "distant" as you seem to think.
2) Alaska and Hawaii, by virtue of their locations, are much more vulnerable to threats on the Pacific Rim (like China) than the Northeast, and that doesn't even take into account territories like Guam which are even closer. Unless you're saying we should cede American territory and American citizens to the enemy, we *have* to be concerned about what the Chinese will do -because they have a much stronger focus on fighting and defeating us than we have in fighting them (take the propaganda films about fighting in Korea or leveling Guam as two examples).
3) If you can't figure out why it's not in our interests to be pushed around by genocidal dictatorships who despise America and would love nothing more than to see us destroyed, whether at their own hands or via internal pressures they heat up...might I suggest reading a history book on what exactly the Germans *did* to occupied Europe during World War II? There's this thing called "The Holocaust" for one.