I said sneak, not stealth. Sneaking a battalion of troops behind enemy lines does not always imply stealthed units in an RTS. I know realism is very very important for you but in an RTS, you can actually bypass enemy lines by hiding them outside of the enemy's LOS.
This is what I meant by sneaking. Besides, who's stupid enough to send even stealthed units directly into an enemy base without fear of detection units? They go around and to be safe, they try to stay away from enemy LOS.