Kind of self explanatory for this solution, all you have to do is complete all of the side missions in the game on any level to unlock this achievement (Veteran difficulty recommended if trying to achieve "The Longest Day" achievement). the following missions are...
Operation Taken Dagger
Operation Phoenix
Operation D-Con
Operation Deep Execute
Operation Safe Harbor
Operation Pure Threat
Operation Grave Robber
Operation Sudden Death
Operation Trace Kill
are all side missions you have access to complete after any mission once back to map/board on the retribution. All you have to do is complete each mission on any difficulty, though i recommend you try veteran difficulty if your pushing for "The Longest Day" achievement.