Basically the best turn-based strategy game there is. Except possibly XCOM. That's no exaggeration, this is easily one of my top five games ever!
As a single-player game, it's better than the other Massive Assault games, but it doesn't have multiplayer. There's a separate game for that.
I'll compare it to two games here:
Advance Wars is a turn-based military strategy game. You build units, you attack with them one at a time, and you try capture cities so that you can build more units.
Hearthstone is a turn-based card game. It's selling point is that every turn is a maths-based puzzle, trying to work out the most effective way to attack.
Massive Assault combines the two. It's a turn-based strategy game with no probabilities and simple figures, so it's possible to work out how a turn will go as soon as you start it. And that's just the START of why it's brilliantly designed!
Oh, and it has undos. Because there's no probabilities or unknowns, you can redo everything up to the start of your turn. Which is excellent!
The big downside is the story and voice acting. The story is just another excuse plot, but at least it's not the main focus, and the voice acting.... oh, it's hilariously bad! Seriously, you haven't lived until you've seen a former soviet try to do an Italian accent!
For the record, at the time it was behind in terms of graphics and sound too. Sound effects, the music is still great. But, like Mischief Makers, now it just looks like a regular old game. Which is good!