First Squad was a group of clone cold assault troopers belonging to a platoon within the 501st Legion. This squad operated under the command of Clone Captain Rex. During the Clone Wars, First Squad participated in the One-Day War against Thi-Sen and his Talz warriors after Chairman Chi Cho of Pantora initiated conflict between the Republic forces and the Talz.
Following unsuccessful negotiations, the Chairman instructed Republic troops to follow him on mounted speeders. Surrounded by Talz warriors, and taking casualties, including the loss of the Pantoran guards and the Chairman sustaining a fatal injury, Clone Captain Rex ordered First Squad to provide covering fire for the Republic forces' retreat.
By the battle's conclusion, eleven clone troopers from the platoon had died, with the remaining troopers, including Chairman Cho, suffering injuries. Once reinforcements arrived and a ceasefire was established, Senator Riyo Chuchi received authorization from the Speaker of the Assembly to negotiate a peace agreement with the Talz, following the condemnation of Chairman Cho's actions. Chuchi successfully achieved this peace.
The first season episode of Star Wars: The Clone Wars, where First Squad made its debut appearance, was the fifteenth episode. It was released in 2009 and directed by Brian Kalin O'Connell.