Can you provide more details? What you've described so far sums up most RTS games. Was it real time? Turn based? Were the units modern day? Historical? Sci-fi?
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Also stuff like Advance Wars
Command and Conquer perhaps? CC Tiberium Wars came out around 2007. Starcraft is also a similar RTS; Starcraft 2 came out in 2010
If it was from 10-20 years ago, top down from an angle with modeled 3d units, it might be one of the Wargame titles from Eugen, or if it was WW2 setting maybe Combat Mission: beyond overlord, Company of Heroes, or Men of War.
If it was straight on top down 2D, it might have been Mud and Blood, which was a WW2 wave defense flash game.
Was it top down in the sense of looking straight down or from above at an angle? Were the units modeled as individual 3d models or just 2D icons?
Also, roughly what time period was it set in? Like, Napoleonic, WW2, Cold War, Contemporary?
Was it single player or multi player focused?
Could you get additional units as the game went on or were you locked with the units you started with? How could you get additional units? Points? Timer?
A lot of real-time tactics and turn-based tactics games would fit that description.
Matrix Games specializes in milsims, so they're an easy way to get a list.
As another commenter says, having more criteria to narrow it down would help.
I like the Close Combat series myself, which is real-time.
Military Madness
?
Pretty vague description, unfortunately. Sounds like it could be one of Eugen Systems' games though?