I was a big fan of the original three XCOM games, especially the third, Apocalypse, which was admittedly very different from the previous ones. They started experimenting with "real time" tactical combat aside the old, turn based one, and there were destroyable elements in the world, providing some extra tactical opportunities for the player. It worked really well for me, but for several reasons I never got around to play as much with it as I'd liked to have.
Fast forward several years. In 2003, UFO: Aftermath is released. The story of the game is basically a typical post apocalyptic struggle, when what is left of humanity fights the invading aliens. I got my hands on it in 2004, and, although I found it mildly interesting, altogether it was too simplistic to keep me entertained. (You can watch a review here.)
Mainly because of this I was quite late to try the sequel, UFO: Aftershock. I can't even recall why I did, I think I read a favourable review, or something like that. Anyway, I did start to play, and got hooked pretty quickly.
In Aftermath, the player is given a choice to choose from different endings. The story of Aftershock continues one of those, where humanity is destroyed (again) and only a small group of survivors remain alive on a space station orbiting Earth. You take control of these people, and land on Earth to find out what happened, and to re-build civilisation. Obviously, it's a bit harder than to plant a few trees..
Unlike most strategy games, UFO:AS requires you to build supply lines to transfer your materials to your bases. But, to make things more interesting, you have to "pay" upkeep for the sections of the supply lines, so you need to organize them in a way that is the most effective. But most effective often means that there is no fallback route, so if something happens (say, the enemy attacks a critical sector) you might find yourself missing some critically important resources, so you need to focus on defence as well as offence.
While on paper this system is great (and in the beginning of the game it indeed is) it becomes a chore by the time when you control most of the planet (a bit of a spoiler: That happens way before the end of the game).
There are similar issues with production and research too. By the time you control the planet, you have a vast amount of research labs, and you need to manually control all of these. After a while it is just too much to do, and very boring as well, but it also gives you a very deep control.
The tactical part is interesting again, the pathfinding and group handling are much better than in the previous game, the maps are smaller that in the original XCOM games (which is actually a good thing, you don't spend hours of real time in a tactical mission). Although I never got around to finish this game, I was pretty close to it, and still think that I should pick the game up again, and play it to the ending.
The reason why I don't do it now is UFO: Afterlight. The story is not happening after the previous part, this time - it happens in parallel. On Mars, there is a scientific station with a lot of personnel. They find some ruins, and on opening them they face some hostile robots. This is where you start with the tutorial.
The strategic part is much clearer and simpler than in UFO:AS, and the story is probably even more interesting, as you delve into the mysteries of a whole new alien race. The research, production, base development are all centralised, and you also have some advisers who will notify you when something interesting happens, or there is an action you could do (for example, I always forget to do the level advancement, but it always warns me to do it).
What I love about this game is that you actually can contact the humans left on earth, choose which side you want to help, and also, you can receive help from them. (it's probably not so interesting if you didn't play UFO:AS, though.) Also, as in the previous installation, you have a few other decisions that will affect the story. I'll talk about this later, in the post where I talk about my own adventures..
To conclude, I like these games, and would like to see an MMO where you play in this setting.