Thanos had finally started to sought out the Infinity Stones in order to cut the population in half to "save" the universe.
The heroes tried everything they could to stop him, but in the end they failed, temporarily.
Having lost so many of their friends in the fight and in the snap, they seeked a plan to bring them back through time travel.
Thanos wasn't going to let them succeed so he intervened, however they succeeded in the end with the sacrifice of Iron-Man.
