It's called United Nations Organization. Although it hasn't been fully implemented. It was planned to have nuclear weapon monopoly and strong joint military forces to stop any aggression. Why didn't that happen? Because it's the US that gained the benefits out of both world wars. And its allies aren't allies but minions. Remember what happened to Charles de Gaulle?

Also note that no one has ever declared a war legally since WW II. Because of the UN and international conventions.

I'd also add that it's not entirely correct to consider countries equal top level actors in the historical process now and in the past. Nowadays so called political nations are technically subjects of international right, of course. But, for instance, in pre-Westphalian world that wasn't the case and these days there is plenty of evidence of transnational actors' influence. For example, Vatican dates back to that era I mentioned. And also who owns most the land in Europe? And how come these von Something German nazis avoided The Nuremberg Trials and ended up as board members in big industrial companies?

So no, the world doesn't need a hegemon in your sense. Taking into account the paralysis of the UN since 1991 it's more likely there will be another take on the ruins that.