That and a ton of spare time and dedication to it. There are those out there that have started on minor projects like this though.
Here's a unity-answers post on it.
Here's a unity3d project copying generals2.
I think the main job would be making the terrain engine and editor for that terrain. I dunno if the inherent terrain class in unity would be good enough for it, so building one from scratch is probably necessary. I don't think that needs to be very advanced though, a grid of 50x50 polyquad grids in the base terrain chunk with a map consisting of from [10x10 to 100x100] of those. Since its grid-based you can implement a a* pathfinder algorithm pretty easily, and make custom "bridges" for a* exceptions with custom mesh like bridges and rocks and whatnot. passable terrain decided by the normal angle compared to a y-vector. if the normal vector minus y-vector is less than 30degrees, you have a less-than 30degrees hill which should be possible for most infantry and the likes.
Anyway, I'm just thinking loudly while waiting for a render here. I'd love to attempt to develop a RTS engine in unity, but I have to maintain pressure on my own projects first. But its not a sin to dream and think about the possibilities