As Bowser says, what if those democracies don't agree with the US?
That's exactly what happened before Gulf War 2 started.
I definitely had the impression conservatives had a better opinion of pro-US dictatorships than of disagreeing democracies.
Those democracies were BTW, following their respective public opinions.

The thing about the UN is that it has a fundamental principle: the notion of non interference in internal matters of sovereign states. It's a notion that exists since 1648 and has a number of times been rejected but except for a few cases (because of the influence of colonizations), has always prevailed.

Many countries are no democracies, granted, but it's up to them to decide to implement it or not; it's not up to foreign powers.
The UN is about countries getting along with each other, in order to avoid wars and other than that, let countries deal with their own internal struggles.

An alliance of democracies? But right here, there is the EU!
What happens? Countries decide to join a huge market, accept rules meant to help poor areas, favor competition, create a single market.
What also happens is a vertuous cycle: a country decides to reduce its military spendings; the neighbours see that and decide to do the same; result: the respective armies become so weak they cannot wage war against each other! And to reconstitute an offensive army, it will take so much time and be so obvious, that the other neighbouring countries won't be surprised and will be able to deal with it.

Raoul