Completed Campaign Mission 27 after completing all of the Armed Recon and Eagle's Nest Missions.
You must complete every available mission in the game before completing the final campaign mission in order to unlock this achievement. All missions will become available to you as you play, except for the Eagle's Nest missions. In order to get them you must find 12 Valhalla reports. Refer to 'Soldier's Medal" for details on where to find the reports.

Note: A good way to get this is to first concentrate on getting the "Victoria Cross" achievement. Save after mission 26, then complete the game. Now you can load that same save and get the achievement. Just complete all the missions you skipped, and replay the final mission.