Why the principles of international relations, which were recognized by all states, could not become a sufficient

Why the principles of international relations, which were recognized by all states, could not become a sufficient guarantee of their peaceful coexistence in the 20th century?

However, for a long time, the principles declared in revolutionary France remained on paper. Events of the 19th century demonstrated that the law of war remained decisive in international relations. And although the history of international relations in modern times knew examples of peaceful international congresses, they first of all secured the right of the winner to dictate his will.