Your talk of UFO and XCOM reminded me of something little bit earlier: Laser Squad (1988).http://www.youtube.com/watch?v=9-G9Z9mJRLM&videos=WeEK9-3mi-o&playnext_from=TL&playnext=1
The video is taken from Commodore 64 version of the game, I played the Spectrum ZX version of it. I don't remember anymore which of the Laser Squads we had on tape. By browsing through these old titles, it is amazing to note that the gaming itself hasn't evolved much from the situation 20 years ago.
Graphics, sounds and save&load possibilities have been added, but quite a lot of core gameplay mechanics have remained the same. As can be seen, the opportunity fire is quite old concept. The bodies are still left on the ground here, and it is possible to loot equipment from them. I think they added possibilites of units being standing (fast movement, poor aim), crouched (moderate movement, moderate aim) and prone (slow movement, good aim) later on some of the game in this series. There is also destructible objects in the environment even in this game, and they could be utilized to the demise of AI!
The morale of the units had quite an effect on everything they do: if they witnessed the death of their team mate, it was certain to affect their actions in the next turn. If somebody carried heavier equipment, they couldn't move fast and had less action points. And so on. Back then I didn't have that much of concept of tactics (being something like 5 years old), but I do remember this game by getting constantly my arse kicked by the AI and wondering what did I do wrong this time?
This is probably one of the oldest turn-based tactical shooting games I recall. By browsing through Wikipedia on Laser Squad, it is actually mentioned that it is also one of the major factors behind UFO, and even working with Space Hulk, which itself is probably a classic, too bad I never got to see it - the only and the best game reviewing magazine in Finland by that time was raving about the game and it being tense as hell.