How might a world conflict eventually lead to greater cooperation among nations?

A major world conflict affects all participants nations so much, that after the conflict ends, they usually have more incentive to cooperate than to engange in conflict again.

Explanation:

This has actually happened in history. After World War II ended, European nations were affected so much, that they decided to cooperate in order to avoid a new war. The main result of this cooperation was the creation of the European Coal and Steel Community, the antecessor of the European Union.

The idea behind was that by linking their economies together, the European nations would have an incentive to cooperate because a war would be even more costly. The idea has been very successful because Europe has not seen a major war like the World War II ever since.