And to this day, it remains the only game in which they do. Final Liberation has quite a few issues(balance favours titans, as vehicles are too weak, there is some "army scaling" involved), but there is really no game like it that would capture the feel of Space Marines(ie, Epic 2nd edition) fights. The game system is a weird hybrid between epic and Steel Panthers, but it works. The cutscenes are pretty atmospheric, being FMV. Slitherine has not been able to reach the quality of this game when it comes to single player campaign. It is also one of the only games in which you can play Imperial Guard with some supporting Space Marines. Highly recommended if you are into 40K!
Chaos Gate oozes with atmosphere: The soundtrack feels like it was made in universe (even though it is mostly repurposed religious chants). Your Space Marines feel like walking tanks. They can still die, and each death of a brother feels painful (especially if playin Mighty Hero mode), but it will take a lot of firepower to bring them down. The game is played over a set of fixed campaign missions, with some randomized skirmishes in between. The campaign missions are pretty well done, and feel like 40Kish X-COM. The skirmish missions are usually much less interesting. Over the course of the campaign, you will unlock a large part of the standard Space Marines roster and armoury (Terminator armors, Librarians, assault marines with jump packs), but you can only bring 3 squads, and need to adhere to the Codex Astartes when it comes to mission loadout (so you'll have more bolters than you'd want!). Even to this day, I still consider it one of the best 40K games made on PC, and I played all of the tactical/strategic ones.