The OpFor achievement says you must win as all three factions in singleplayer on hardcore. I have done this two times now and I have not gotten my achievement. What is going on?

Its glitched there are alot of things you have to do in order to get it. BTW if you dont believe me i have the achievement.
1.You must play through hardcore as each faction but save before the last mission on all three.
2. Then you need to go clear your cache and perform maitenance on your xbox so it clears out all of the xbox live updates.
3. after you do that put endwar back in make sure you dont update it(it will make you play offline.
4. Then finish the campaigns as each faction OFFLINE and THIS IS IMPORTANT watch all of the credits after each play through.

you dont need to do prelude to war for any of the factions, but be sure to watch all the victory scenes after each play through, as well as the long and boring credits.

Hope this helped

well i didnt get mine first time round and then i went and did all the prelude to wars first so it took me staright into world war 3 with that faction and i unlocked it