War BondsG 15

War Bonds G15:
You have had at least 3,000,000 Command Points.

The achievement says that you must hold 3,000,000 CP at one time, not cumulative. Depending on if you bought the Double Command Points perk in the store, this might take you a while. By completing missions and surviving in the zombie maps, you earn CP. You earn more CP by completing the secondary objectives in missions. Repeating missions that you are comfortable with makes it go faster.