I agree with all that has gone before, and would add that as I see it there are two major barriers that are overlooked when dealing with PKO. First is the fact that Blue Helmets come from all over the world, have different missions, different rules of engagement, and most importantly speak different languages. The second problem is that because of the way that the UN creates Blue Helmet "armies" creates and incentive structure whereby countries who cannot afford to equip and train their militaries are the ones who have the most to gain by sending them on Peace Keeping mission, to get them equipped and at least a little trained, not on their dime.

The illustration to this problem is the Eastern front of WWI. The Austro-Hungarians, who basically commanded a large coalition of Armies made up from the various Hapsburg holdings were creamed by the Russians, who mostly spoke Russian, and were far more unified in comparison. The Russians, in their turn, we trounced by the Germans, who were not only more unified than the Russians, but also well trained and well equipped.

I know that this is a simplification of WWI, but the analogy holds.