*list*
Dawn of War (haven't played Dawn of War 2 yet) is an RTS game. Granted, due to its squad and vehicle limits it plays more tactically than "spam mammoth tanks and win" C&C's. But you're still building bases and recruiting units. Fairly good RTS though. Worth recommending.
Likewise I would disagree with Commandos. It's more of a puzzle game in set-piece levels, rather than a tactics game. Good games if you're into the genre, but rather different from the likes of x-com and it's ilk.
Rest seem fitting recommendations though.
There's this game called UFO: Extraterrestrials (no relation to UFO: After-foo series). It's a -07 game from east europe so not exactly a big name game. It's fairly close to X-COM, theme and everything. Tactical combat, off-mission base building, it's all there.
Not exactly what you're looking for, but how about Rainbow Six series? It's tactical squad based combat. Only you're one of the squad members (can jump from member to member freely) and it's real time. You're a counter-terrorist unit that hops the world saving hostages, diffusing bombs etc.. Really good games at first (don't even mention the console crud or vegas stuff, ugh). The games you want are Rainbow Six, Rainbow Six: Rogue Spear (aka. Rainbow Six 2) and Rainbow Six 3. All have one or more expansions which are basically more of the same. Anything past Rainbow Six 3 is a whole different genre of FPS that for some reason has the R6 name attached.
Edit:
In the same vein there's the Ghost Recon series. More military, less counter terrorizing. Somewhat less urban environments from what I remember too. Pretty similar gameplay though.