Surely, you are not that naive.
No, but it's a practical idea, if we're looking at this issue strictly from our own pragmatic national interests.
As far as being naïve, that's in the eye of the beholder. I've always found it fascinating how conservative warmongers during the Cold War would chastise so-called "bleeding heart liberals" for wanting to help people here at home. Even today, I hear how "Social Justice Warriors" are lambasted for much the same reason.
Yet, all of this is somehow reversed when it comes to geopolitics. Then, suddenly, the heartless capitalists and conservatives inexplicably start caring about others, as long as they're on foreign soil. They're the SJW's and bleeding hearts of geopolitics. They care nothing about the homeless on the streets of America, but when it comes to some helpless people on the other side of the planet, suddenly they care.
Do you think I'm so naïve as to not be able to see through that?
How long did it take before US decided to take action in WWII?
FDR wanted to take action sooner, but faced a great deal of opposition from those who wanted to stay out of it. There are several reasons for this, some of which go back to the principles upon which America was founded.
Isn't it preferable to stop countries from military aggression against their neighbours, before it gets out of control?
I suppose a case can be made for such, but it largely depends upon the circumstances and the context. For example, the Franco-Prussian War of 1871 was a quick six-week war which didn't get out of control. It ended quickly, with the capture of Napoleon III and the annexation of Alsace-Lorraine by Germany. Maybe some other country might have stopped Germany, but the fact that no one did, that didn't lead the situation to get out of control. Europe and the rest of the world have gone through many, many wars - most of which did not require the US military to come in and restore order.
Of course, another question might be asked: Isn't it preferable for countries to be able to defend themselves so that they don't have to worry about military aggression from their neighbors? This is not an unreasonable question, as both Britain and France had sizable populations, modern industries, and huge empires teeming with resources and manpower to be able to wrap up Germany rather quickly, without help from either the Russians or the Americans. Why were they caught so woefully unprepared?
This seems to be a common narrative, where the US points out some "bully" who picks on some weaker "pipsqueak" of a nation, thus turning the US into a white knight to come in and rescue the damsel in distress. But why is the damsel in distress in the first place? Nobody ever asks that question.
Why is Ukraine not strong enough to defend itself? Especially if they're so worried about being bullied by their larger neighbor. Russia is certainly not invincible, and they are nowhere near as strong as they were during the Soviet period. So, why should we have to get involved at all? Just because other countries don't want to lift a finger in their own defense?
Are you saying that NATO is defunct .. unnecessary?
The reason for forming NATO was to protect Western Europe from the Soviet Union and the potential spread of communist revolutionary ideology. It was purely ideological. Once the Warsaw Pact disbanded and the Soviet Union broke up, the original pretext for NATO's existence was no longer relevant. It should have been disbanded in 1991, but the fact that it was not could be interpreted as an act of aggression against Russia, which peacefully and unilaterally chose to stand down in good faith. They acted in good faith, but our government did not. That's why the Russians have grudge against us now.