And before Steel Panthers was even a glimmer in SSI and Grigsby's eyes:
First, you had KAMPFGRUPPE in 1985:
http://www.mobygames.com/game/kampfgruppe
East Front 1941-45
and MECH BRIGADE, again in 1985:
http://www.mobygames.com/game/mech-brigade
NATO/USSR conflict.
followed by
PANZER STRIKE in 1987
http://www.mobygames.com/game/panzer-strike
Basically, East Front, North Africa, West Front (with no americans).
Then TYPHOON OF STEEL in 1988
http://www.mobygames.com/game/typhoon-of-steel
Which had PTO (US v Jap, UK v Jap) and the remaining ETO stuff (US v GER).
https://www.youtube.com/watch?v=qbLmIhQnWvY
Youtube of the DOS EGA version -- the C64 version was programmed by Gary Grigsby.
All of these games were semi-turn based / real time hybrids, similar to COMBAT MISSION, in which you gave your orders and then let the game units execute them, but you can see the definite influences that they had on Steel Panthers, as Grigsby likes to reuse/recycle basic informational concepts.