United Nations Security Council has the power to impose sanctions, including
the use of armed force
against a nation-state, or even against a faction within a nation. Intervention
is possible against a nation which is determined to be threatening the
peace, or against a state or faction within a nation which is determined
to be in gross violation of human rights.
The membership of the Security Council consists of 15 member states, 5 of which are permanent members, and the other 10 seats are rotated among the general membership of the United Nation. Decisions are by majority vote, with each of the 5 permanent members possessing the power of an absolute veto. The veto must be actively asserted. That is, abstention from voting is not a veto.
When the United Nations was formed in 1945, the five great powers; the United States, the Soviet Union, the United Kingdom, France, and China, would not give the Security council the power to establish sanctions without ensuring that they each had a veto. The veto power gives any one of the five permanent members the ability to paralyze the Security Council.