United Earth Space Corps
From Halopedia, the Halo wiki
The United Earth Space Corps is a human organization. Fleet Admiral Terrence Hood addressed Sergeant Major Avery Johnson as "a soldier of the United Earth Space Corps" when awarding him the Colonial Cross. The UESC's relationship to the United Nations Space Command and Unified Earth Government is unclear.
It is possible that "United Earth Space Corps" is simply another name for the UNSC, perhaps used in ceremonial occasions to highlight their allegiance to Earth. The name may instead refer to elements of the UNSC tasked with the protection of Earth. Such a force would likely include elements permanently stationed on and around Earth.
Alternatively, the UESC may be a local defense force separate from the UNSC. As the UEG's member nations continue to maintain their own militaries as late as the mid-26th century, the UESC thus may serve as an aggregation of Earth's national military forces. However, there is no evidence that Avery Johnson served in any military before enlisting in the UNSC Marine Corps at the age of 19.
There is also the idea that the UNSC deployed its marines for the first time in 2163, during the Mars Campaign and before even the UEG, suggesting that at is creation, it was still an organization under the United Nations. When Earth joined together as the UEG, the UNSC was most likely turned into the UESC officially, but still called the UNSC casually as a rememberance of its formation. 
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