Mass Effect: Paragon Lost

During the early days of James Vega, he lead a squad of elite special forces into a battle against the mysterious Collectors. It's the squad's mission to ensure the safety of the inhabitants of the remote star system from the alien threat in which they want to collect the inhabitants for unknown purposes. After the attack against the collectors, Vega's commanding officer became a casualty of the battle, and this lead to Vega becoming the new leader for the squad. Eventually, this lead to Vega to make some hard decisions that can alter the lives of the squad and the population of the colony.

Synopsis from Anime News Network