The United Nations Security Council is effectively the executive arm of the United Nations when negotiating or implementing directives for the Earth and its directly controlled colonies. It has the power to direct the Earth's missile arsenal, command the United Nations Navy fleet, and direct the United Nations Marines force.
Unlike in the past (a few hundred years earlier), the Security Council has changed structure to be primarily run by the Secretary-General, Under-Secretaries, UN Navy generals, and advisors. In the past it was controlled largely by permanent UN Security Council member states: China, France, Great Britain, Russia, and the United States. It still is the real power in the UN and even more so as humanity's reach has expanded.