Close Combat is a mostly highly regarded real-time strategy series that began in the 1990s. They were pretty hardcore, taking into account cover and suppression, fatigue and morale, among a range of systems that went deeper than other popular RTSs of the era.
Three of those games from the original series are now available on GOG: Close Combat 2: A Bridge Too Far, Close Combat 3: The Russian Front and Close Combat 4: Battle of the Bulge.
Curiously, there's no sign of the first game. But to be honest, A Bridge Too Far is by far the best in the series, so I wouldn't worry too much.
GOG has also nabbed two more recent Close Combat games, 2012's Panthers in the Fog and 2014's Gateway to Caen. They're okay and, unlike the other three, have been available on Steam for several years.