The basis of their existence was to prevent war among the major powers, for which it was pretty much successful. But they can't control everything all over the world.
The main trouble is that we've set ourselves up as the ones to "make the world safe for democracy," a policy statement which has made us into an international ideological enforcement agency. But we haven't been terribly consistent in that regard. All these decades, we've claimed to be fighting tyranny in favor of freedom and democracy, yet we've supported regimes which are/were decidedly "not free" or "partly free" (using Freedom House standards).
As a result, our policies come off looking inconsistent, incoherent, and/or hypocritical - which has triggered some violent reactions out there and created conflicts. Where we've truly failed is in the fact that we've compromised and tainted our role as mediator, since many factions correctly see the US as no longer being neutral in global affairs. (Contrary to popular belief, the US was never really "isolationist." We were neutral. Big difference. But we're not neutral anymore, and this has become a problem.) Our policymakers have been so flaky and inconsistent that no one trusts us anymore. Even those who have been our close friends and allies - even they wonder about us.
I don't think this means that NATO and the UN are helpless to stop ISIS, but the problem is obviously political. Intervening in another country's civil war is always tricky - and there doesn't appear to be any right answer here. The UN is committed to the sovereignty of independent nations, even where it ostensibly contradicts or conflicts with the UN declarations on human rights.
It seems like it might be easier to marshal our combined forces and make an all-out, no-holds-barred attack on ISIS and their strongholds, as long as we didn't have to be too dainty about how we did it. But we're bound by international treaties, so our hands are tied as to how far we can actually go. Some US politicians say we should "bomb them back to the Stone Age," which could probably work to stop ISIS, as long as we're willing to deal with the ramifications of wiping out an entire country.