campaign question

you have to go back and beat it all on veteran at one sitting.

I should work, but I have always played the game on vet to start.  But I do remember playing an older cod game where I did the same thing and I had to complete the whole thing again to get the achievement.  But I would imagine games are smarter now and will know.  

Hmmm, good question. I dont do campaigns, but I still learn a good bit about them from the forums, but I dont know this answer. I would think you could switch around from mission to mission, beating the game on easy, then be able to go back to each mission and adjust the difficulty to vet, then complete it that ways, as you say you have done, dunno fo sho tho.

The game will be able to recognize what missions you have completed on veteran. It is like Halo in a way, it looks at your campaign progress and the missions completed, by the highest difficulty completed.