The Syrian civil war was initially a conflict between Assad, and the Free Syrian Army, into which NATO poured munitions, training, and 'volunteers' from the Lybian civil war, letting it blossomed into a multi-directional hellscape, involving Assad, the FSA, Kurdish groups, and notably, Al Qaeda's Syrian branch, Al Nusra (Which, ironically, is considered by the West to be a 'moderate' group in the conflict. We've done a good job of bombing the people they've been fighting against.) Soon after that, ISIS, the estranged lovechild of the Iraqi civil war enters the picture.

No, Syria wasn't invaded by NATO - instead, NATO poured, and continues to pour oil on that fire, because instead of making the situation better, it would rather fight a proxy war with Russia.

Despite our best efforts at supporting the rebel groups, and bombing Assad, he's still in power. Given that, I think it's reasonable to conclude that had the conflict been left to resolve itself, he would have won fairly quickly - or at least, by now, most of the country would be under his control, as opposed to a five-way horror-show. We'd also probably be seeing a lot fewer beheading videos on Twitter.

NATO's perspective on the whole conflict, start to end, was not humanitarian - but rather as a question of "What can we do to stick it to the Russians, and topple Assad?"