I just reccommend you don't use this idea, it might sound cool, but its been done a hundred times. From the Jedi, Spectres, Rainbow 6, Ghost Recon, even this 'The Losers' movie looks similar in some ways.
It is impossible, and if you try to write it as anything but that way then you're going to either be ripping someone else off, or its just going to be so false it will be a joke. I'm not trying to be cruel but its true. The idea is impossible, for SO many reasons. If they are led by the UN then there are still going to be biased, especially since the soldiers will be from countries. For example, if you pull from the best special forces in the world? Its mostly going to be British, Israeli and American. Americans believe they have the best in the world so they'll want to be the main spot light in a story (hence why rainbow 6 worked decently). So for example....you'd make the Americans the majority, but wait a second....people believe that what George Bush did with invading Iraq was a war crime. Then the team should go in and get the American president. Good luck with that. Such things are impossible, biased factions and commanders would make it totally impossible, and of course, as I said before, the differing opinions and views on things.