From the Steam store page:
XCOM 2 is the sequel to XCOM: Enemy Unknown, the 2012 award-winning strategy game of the year.

Earth has changed. Twenty years have passed since world leaders offered an unconditional surrender to alien forces. XCOM, the planet’s last line of defense, was left decimated and scattered. Now, in XCOM 2, the aliens rule Earth, building shining cities that promise a brilliant future for humanity on the surface, while concealing a sinister agenda and eliminating all who dissent from their new order.

Only those who live at the edges of the world have a margin of freedom. Here, a force gathers once again to stand up for humanity. Always on the run, and facing impossible odds, the remnant XCOM forces must find a way to ignite a global resistance, and eliminate the alien threat once and for all.