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 beggining of Halo 3 when a Sergeant says "second squad coming through" when you are about to fight the first group of Covenant which consisted of grunts and jackals

[2]