But perhaps it really is always 1939. Or, to be even more glib, what you are saying sounds to me much like:

Every time we see icebergs on the horizon, you steer around them, but the ship never actually sinks. And every time I say we should forget about icebergs, you bring up the Titanic. You paranoiacs really need to get a new routine, people aren't falling for it anymore.

OK, let's be serious. Let's say that "being Hitler" means going on an ever-increasing campaign of conquest against neighbouring countries that results in a very damaging war. We could note that this kind of behaviour was very common prior to the Napoleonic Wars (Napoleon, Frederick the Great, Charles XII, Louis XIV, Wallenstein, Philip II, Suleiman I, etc etc). Since the Napoleonic Wars, there have been a number of international frameworks more-or-less explicitly devoted to prevent "Hitlers," and which have had some success. However, IR is basically anarchy, which means that when actors fail to abide by the rules of those international framework, forcing them in line means war.

There haven't been a lot of "Hitlers" in recent years. But at least part of the reason that the people you call "warmongers" nipped the likes of Saddam, Milosevic, Galtieri, etc in the bud. For example, Saddam was definitely on the Hitler path in 1991, and what stopped him was western military intervention. And not only did this intervention stop him, but it acted as a warning to other leaders who might be considering more bellicose action, and helped reinforce the rules and norms of our peaceful international framework.

But this commitment is forever being tested, because leaders get glory through war. Consider that the fame and popularity of American Presidents has been shown to be higher the more Americans who die in military combat in their term in office (!) and then multiply that for more bellicose societies such as Russia. A peaceful world requires constant vigilance.

In the current case, it's clear that Putin is engaged in an aggressive campaign of conquest against his neighbours (not merely in Ukraine). It's not clear where this will stop. It's clear that every victory strengthens Putin's domestic position and emboldens him for the next step. This is the classic "Hitlerian" path, and everyone from Hillary Clinton to Prince Charles has noted it. That doesn't mean the West should necessarily intervene (maybe the costs are greater than the benefits) but it's pretty obvious why the comparison is so widely made. I'd say the 'routine' that no-one is falling for is the Panglossian one that Putin isn't a predator - with each new Russian outrage, it becomes harder and harder to sustain.

I haven't heard the (potential) interventions against Qaddafi and Assad justified on the same grounds. There, the justification is usually a combination of (1) humanitarian and (2) preventing failed states, which is rather different.