The Global Defense Force, or GDF, is a supranational paramilitary
organization, formed in the mid-21st Century from a large pool of national military organizations and relief agencies, to combat increasingly severe weather events, other natural disasters and terrorist campaigns, as well as to keep the peace in historically unstable regions such as the Middle East. It was created shortly before the Strogg invasion of Earth.
The GDF is a fencible military, as opposed to the Space Marine Corps which is expeditionary.
Containing a large proportion of Mankind's military and relief capability at the unexpected outbreak of war, it was the only military organization on Earth with the equipment, command structure and mobility to effectively combat the sudden emergence of the alien menace, so the task of global defense naturally fell to it.
Over the long years of the Strogg War on Earth, the GDF would see much of the remaining military capacity of Earth falling under its purview as nation after nation fell prey to Strogg armies, bombardments and attempted colonization. Towards the end of the War on Earth, it would form the Terran Coalition of Man, specifically with the intent to launch a counter-invasion of the Strogg homeworld.