When Hitler took over his party, it was a ragtag tiny collection of people who virtually nobody was paying any attention to (one of many in Germany at the time).
History is not repeating itself. We really need to stop looking at the 1930s as our guiding star for international relations. 2015 is not 1939, nor does it resemble 1939 in anything other than the most crude and superficial ways.
Now, if we were to intervene against IS, what would happen?
It would be defeated. Probably quite easily. Then, eventually, we would go home. And then what?
After we left, everyone would know that its former territory in Iraq was protected by a military force which fled from a group which at the time did not number more than a few thousand.
I know that intervening would make some of us feel a bit better in the short run, but in my opinion it would achieve absolutely nothing and we would be back to square one within a couple of years. "The world" (by which you effectively mean us and the USA) cannot solve this problem. At least not by simply invading.
The other alternative, of course, would be to occupy the area again and develop it in a positive direction. This is what we tried to do after 2003, and the public quickly disowned it within a few years. I think it's pretty likely that the same thing would happen if we tried it again now.
I know this won't be popular, but there isn't much we can do. The situation in the middle east is going to get a lot worse over the next decade or so, and we blew our chance to do anything about it years ago. We're in for a less secure, less stable world for a few years at least.