After the human settlements a.k.a. the utopia was built in different countries, few wars were declared against the U.S. Sector for their supplies and weapons, so to fight back, the U.S. Sector had developed their own military.
Many men and women were enlisted and sent to battle not only against the humans in other countries, but the undead that crossed paths with them. Although many were enlisted, most of them did not survive the required special and dropped out.
Existing soldiers today are specialized in many deadly combat skills and weapons. These men and women are only to serve the U.S. Sector Council members and nobody else. On special occasions, the soldiers are allowed to retire for good before they hit their 50s (This is very rare), but they are under surveillance by the Council and are expected to act when their service is needed with no other option.
During the Beta run, only a few soldiers were used to clear the city due to their high demands to serve the Council members. To their surprise, all of the soldiers did not survive the Beta run of the game. To avoid the same incident to happen, the Council members agreed to ban the use of soldiers for the Dead Game for now, this includes Veterans.