I'm assuming you mean military in nature, because of course all sorts of aid agencies have been successfully saving and aiding people for decades.

I'm not sure what you think the scope is. The U.S. involvement in Europe in World War 2 would count, no? But let's aim for post-Cold-War.

There's a good argument to be made that NATO's intervention in Bosnia prevented a genocide. After having mucked stuff up, the additional intervention to crush ISIS in Iraq was arguably successful. The intervention in East Timor also seems to have saved far more lives, and preserved a more functional social situation, than leaving the situation alone would have. ECOWAS intervention in Liberia in the 90s was not without problems, but probably was net positive, and the 2017 deployment of ECOWAS/Senegalese troops to Gambia was likely the better outcome though things never got bad enough to be sure. There is widespread agreement that not intervening in Rwanda more forcefully allowed the genocide to continue (sentiment is that it could have been stopped).