It's about 2 years into the War on the Frontier, after Project Alamo introduced humans to the farthest reaches of Space, some took advantage of the program, and tried to Overtake the Entire Frontier.

The Peacekeeper Forces of Earth had to Defend the Frontier, and thus the Sentinel 4 was created. Which took the most elite and prepped them for war on the Frontier

Your entire squad was killed and you've been transferred to Clementine's squad.

Clem looks up from her data pad, and she nods