XCOM: Enemy Within is one of the four new games added to the program today.
For the second time this week, Microsoft is adding more Xbox 360 games to the Xbox One's backwards compatibility catalog. The new additions for today, June 9, include XCOM: Enemy Within, Foul Play, Monopoly Plus, and Sega Bass Fishing.

As usual, Microsoft's Larry "Major Nelson" Hyrb announced these on Twitter.
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Earlier this week, Microsoft added Aqua, Anomaly Warzone Earth, and Lazy Raiders to the backwards compatibility catalog