To unlock, you need complete the Allied campaign and the Soviet campaign. I included a walkthrough of every missions on hard difficulty below:
ALLIED CAMPAIGNMission 1: In The Thick Of ItMission 2: Five To OneMission 3A: Dead End (North)Mission 3B: Dead End (South)Mission 4: Ten To OneMission 5A: Tanya's Tale (North)Mission 5B: Tanya's Tale (Center)Mission 5C: Tanya's Tale (South)Mission 6A: Iron Curtain Infiltration (North)Mission 6B: Iron Curtain Infiltration (South)Mission 7: Sunken TreasureMission 8A: Protect The Chronosphere (North)Mission 8B: Protect The Chronosphere (South)Mission 9A: Extract Kosygin (North)Mission 9B: Extract Kosygin (South)Mission 10A: SuspicionMission 10B: EvidenceMission 11A: Naval Supremacy (North)Mission 11B: Naval Supremacy (South)Mission 12: TakedownMission 13: Focused BlastMission 14: No RemorseSOVIET CAMPAIGNMission 1: Lesson In BloodMission 2A: Guard Duty (North)Mission 2B: Guard Duty (South)Mission 3: Covert CleanupMission 4A: Behind The Lines (North)Mission 4B: Behind The Lines (South)Mission 5: Distant ThunderMission 6A: Bridge Over The River GrotznyMission 6B: Bridge Over The River VizchGoiMission 7: Core Of The MatterMission 8A: Elba Island (North)Mission 8B: Elba Island (South)Mission 9: Liability EliminationMission 10: OverseerMission 11A: Sunk Coast (North)Mission 11B: Sunk Coast (South)Mission 12: Capture The Tech CentersMission 13A: Capture The Chronosphere (North)Mission 13B: Capture The Chronosphere (South)Mission 14: Soviet Supremacy