"When R.U.S.E. is released in September, it will benefit from Valve's Steamworks API to offer the best community experience to players. Consequently, a Steam account and Internet connection will be required to activate the game, as per Steam policy," Ubisoft community developer Aymeric 'Wuzzi' Evennou explained. "For this reason, R.U.S.E. will not use the Ubisoft protection. Single player can be played offline."
While it's been known for some time that R.U.S.E. is using Valve's Steamworks, it was unknown whether Ubisoft's own DRM would also be used or not.
Ubisoft's DRM solution requires players to be online at all times, pausing the game should they lose their connection to the authorisation servers--meaning the game cannot be played offline. The DRM has also suffered server problems in the past, disrupting the games of some players and leaving others unable to play at all.
R.U.S.E. is slated for release on PC, Xbox 360 and PlayStation 3 in September, having been delayed from a planned June 8 launch to build upon public beta test feedback.