The unlimited right to start, initiate, and wage war, and use force against another state.
Also jus belli, or the law of war (an obvious oxymoron).
In International Law (Wallace):
"Jus ad bellum is the right to wage war, a right initially enjoyed by sovereigns.
"The right to wage war is no longer regarded as a legitimate instrument of national policy and is prohibited under contemporary international law."
The primary modern restriction on the use of jus ad bellum is at §2(3) and (4) of the UN Charter:
"All Members shall settle their international disputes by peaceful means in such a manner that international peace and security, and justice, are not endangered.
"All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations."
Two exceptions linger on in international law: the first is the defensive war or the war for just cause.
In addition, somewhat paradoxically, international law has a body of law to set out the rules of war once initiated, such as the Geneva Convention on the Treatment of Prisoners.
REFERENCES:
- Duhaime, Lloyd, International Law
- Duhaime, Lloyd, International Law Dictionary
- Duhaime, Lloyd, Latin Law Dictionary
- Wallace, R. and Holliday, A., International Law, 1st Edition (London: Sweet & Maxwell, 2006), page 121