But here's the thing about the United Nations: it's most basic mandate is not to protect people... at least not directly. Its most basic mandate is to "maintain international peace and security," or, put another way, to prevent war between states.
That's not surprising: when the U.N. was founded in 1945, international aggression was on everyone's minds. And though we often forget it, the system does what it's supposed to: direct, hot wars between states have become exceedingly rare. Instead, we have internal rebellions, proxy wars, and insurgencies. The U.N. isn't really built to deal with these, for three understandable reasons.
1) The U.N. respects the sovereignty of states. Because Russia and China trot out the word "sovereignty" so often to defend abusive regimes, the word itself has become befouled. It shouldn't be. Respect for sovereign borders undergirds the long peace we've enjoyed since the U.N.'s founding. Territorial conquest in the 21st century is so rare that we're shocked when it happens. Russia's seizure of Crimea was the first time a great power annexed territory by force in more than half a century.
2) The U.N. respects great powers. The five permanent members of the U.N. Security Council (the U.S., U.K., Russia, China, and France) can veto anything, so the U.N. is powerless to act if any two of them fundamentally disagree. But this isn't a bad thing either: the Council reflects geopolitical reality, so its weakness is also its strength. The very dynamic that makes the Security Council unable to act on Syria allows it to impose sanctions on Iran; secure a just electoral result in Cote d'Ivoire; mediate Kenya's post-election violence; and keep Cypriots from warring. If you can get all five permanent members to agree on something, it has power and legitimacy you can't get anywhere else. (Also, if there were no veto, Washington and Moscow, at minimum, would have walked away a long time ago, rendering the U.N. irrelevant.)
3. The U.N. doesn't like to take sides. This is a good thing as well, because impartiality is critical to the organization's legitimacy. U.N. peacekeeping is at its best when combatants are tired of war but don't trust each other enough to create peace. Blue helmets are pretty good at keeping a peace that couldn't exist without them... but very bad at making a peace by defeating one or more combatants. Sending U.N. or U.N.-authorized troops to be a proxy army in Somalia (see: AMISOM) or to help a dubious national army defeat equally dubious rebels (see: MONUSCO) sullies the U.N.'s impartiality and generally doesn't work, since U.N. troops are usually less well-armed and less motivated than in-country combatants. (It also means that, increasingly, U.N. forces are deliberately targeted. On many battlefields, they are now, often rightly, viewed as favoring one side over another.) U.N. troops are pointless when they're just another shark at a feeding frenzy.
It's not satisfying, as we mark the 20th anniversary of Srebrenica, to consider why the United Nations is still ill-prepared to stop another Srebrenica, but it's important. Because the U.N. is built first and foremost to keep the peace between states, it struggles when those states fall apart. But I'm not sure we'd want it any other way.