Not to mention that the US is fond of breaking treaties left and right, though with the current administration it would surprise me if they didn't jump at the chance to attack anyone anywhere for the least provocation.

NATO does oblige all members to come to the defence of another member in response to an attack.

This is a good point, and I have an idea I want to toss out here- maybe what's really dangerous is to have a mutual defense treaty between a lot of states but still allow them to have separate diplomatic relationships.. so that a snafu involving one member state can blow up and involve a bunch of countries who didn't need to be involved. If there was a central agency speaking for all those states then there would be fewer ways for a disagreement to occur. So what I'm saying is that 20 small countries will a mutual defense treaty are much more likely to be involved in a war than one big country over the same area. I don't know if that's true or not- you'd have to do the number crunching to find out..