Some have mentioned in the comments that they didn't need to complete Ashes to Ashes if they didn't chose that ending. For me I did then it poped so your milage may vary
I would like to echo the other guides in saying it is to beat the campaign on Veteran.
But in order for the achievement to pop you must complete both missions, Ashes to Ashes and The Final Countdown. The choice for the different missions to either
is found in the mission Identity Crisis.
Another side not is you DO NOT need to complete the side missions for this to unlock. I had only completed one (Red Circus) during my Veteran play through and still got the pop.
Hope this helps, Happy Hunting.