From Command and Conquer Wiki

The United Nations Military Command is a military organization formed in 1995, which consisted of the UN peacekeeping forces and the Global Defense Initiative. General Mark Jamison Sheppard served under the UN Military Command as GDI Chief of Staff. During the First Tiberium War, United Nations Military Command Center served as the organization headquarters.

With ecological devastation caused by Tiberium, the UN faded into irrelevance. The UN Military Command was eventually absorbed into the GDI United Peacekeepers.