Second Squad
From Halopedia, the Halo wiki
Second Squad[1] was a United Nations Space Command Marine Corps military unit.
It was stationed as a perimeter patrol for aboveground vehicular hangars GG and HH during the Second Battle of Earth in late 2552. In addition, they are in the beginning of the level Tsavo Highway in the Halo 3 campaign when Sergeant Reynolds says "second squad coming through" when you are about to fight the first group of Covenant which consisted of grunts and jackals.
