Zimbabwe is self-destructing. Other than providing a bad example that other like-minded tyrants might want to follow (as some South African activists advocated in the recent conference), Zimbabwe poses little threat to other nations.

Algeria has its problems, but do they sponsor international terrorism? Do they attack their neighbors?

So many tyrants, so little time. We can't take them all on at once, so we have to prioritize. Those that pose a threat to the US and our friends are higher priority than those that simply are destroying themselves.

As for Europe, the devastating losses they suffered in WWI apparently made them too timid to stop the threat posed by the dictators. The end result was even more devastating losses in WWII.

Actions have consequences and so does inaction. A few years ago, the spectre of genocide once again raised its head in Europe. Apparently, Europe was too weak or too timid to deal with a problem in their own backyard, so the US had to deal with it. You can have all the common currencies and the like that you want, but if you're too weak (or timid) to deal with your problems, then you have a problem.