Obligating all states not to interfere in any way in the internal affairs of other states.
Non-intervention came into being after the French Revolution when the new government was attempting to establish a non-royal form of government and did not want the interference of royalist sympathizing countries. The USA at the same time used non-intervention as the rationale for the Monroe Doctrine which was directed against intervention by imperial Europe in the Americas. The UN supports this principle.
The strategy for non-intervention requires stable national states that can function independently.