Yes, [MENTION=6966824]Esker[/MENTION] and I are walking the same road here. 
I think 2 levels of fighter are essential, as are the four levels of rogue. The other 4 levels of fighter are cool and come with great toys, and I would certainly go that route, but they are less essential to the build IMO, provided you are going to pick up a second attack somewhere else (Ranger, or whatever).
The question of where to start is interesting but not actually super critical. I'd never advise someone to not start rogue if they wanted that extra skill or thought it was essential for their build. That might be a big deal for a campaign that was going to be third pillar intensive, for example, which would mitigate for all the social skills you can get your hands on.
The nice thing about the assassin is depending on what kind of character you're building you can go just about any direction. A old-school assassin- paid killer, disguises, leaves a folded paper crane on every victim - can be built by just staying rogue. But you can do a heap of other fun stuff. Personally, I like a little magic in my assassin builds, and there are a number of ways to make that happen, Gloomstalker being a personal fav, but Sorcerer, Warlock, and Bard all have their MC uses. Ranger sounds like it might fit the vaguely special ops military feel of the OP, but you can really let your imagination run wild.

I think 2 levels of fighter are essential, as are the four levels of rogue. The other 4 levels of fighter are cool and come with great toys, and I would certainly go that route, but they are less essential to the build IMO, provided you are going to pick up a second attack somewhere else (Ranger, or whatever).
The question of where to start is interesting but not actually super critical. I'd never advise someone to not start rogue if they wanted that extra skill or thought it was essential for their build. That might be a big deal for a campaign that was going to be third pillar intensive, for example, which would mitigate for all the social skills you can get your hands on.
The nice thing about the assassin is depending on what kind of character you're building you can go just about any direction. A old-school assassin- paid killer, disguises, leaves a folded paper crane on every victim - can be built by just staying rogue. But you can do a heap of other fun stuff. Personally, I like a little magic in my assassin builds, and there are a number of ways to make that happen, Gloomstalker being a personal fav, but Sorcerer, Warlock, and Bard all have their MC uses. Ranger sounds like it might fit the vaguely special ops military feel of the OP, but you can really let your imagination run wild.