The UN Security Council can direct military force.
Is the decision making of the Council by unanimity of member states, or is there a supranational decision-making body that can direct force without needing to consult nation states?
Article 24 of the security council mandates the council to act on behalf of the national members of the United Nations. It consists of 15 nations, 5 are permanent (US, UK, France, China, Russia) and 10 which elected by the other members of the UN. These countries are represented by ambassadors. Ambassadors have to follow the instructions of their country; they can't act independently of their government.
It may authorise the use of force by one Nation against another (for example the use of force by the USA and others against Iraq following the 1991 invasion of Kuwait). It can also call for the formation of a peacekeeping force (the blue berets) drawn from the armies of various nations.
There is no other body that the various nations recognise as having the authority to permit military action. There is no "world army" or any other body that can direct military force that is not composed of national representatives.
The security council does not operate on the basis of unanimity but instead requires a 9:6 supermajority including no veto from any of the five permanent members. All security decisions of the council must follow this process, thus to authorise force, all of US, UK, France, China, Russia must agree to it, and another four members of the security council must vote in favour.