After world war ii, many countries believed in the principle of collective security, which means that

After World War II, many countries believed in the principle of collective security, which means that nations should act together to promote peace.

Explanation:

Collective security is based on states giving part of their security policy sovereignty to an international authority. Peace must be ensured by the states acting jointly instead of individually and solely for self-interest.  Supporters of idealism will often believe that collective security is the most optimal security system in terms of maintaining world peace. The UN is an example of such a collective security organization.

The certainty is that any international war risks are reduced if the system otherwise works. However, such collective security systems have never worked 100% as intended. One of the biggest problems is that major powers in particular are unwilling to surrender their full sovereignty over military means. This includes the reason why the UN has a Security Council, where five of the world's great powers have the right to veto military decisions.