After being critically wounded in Vietnam, Dr. Burstein used his variation of the Super-Soldier Serum on Tanner to save his life. The result was granite like skin and super-strength. In some regard, Burstein sees Tanner as an "older brother' to his later test subject Luke Cage. Perhaps the stress of seeing his hone village in Vietnam being destroyed and his wife being murdered or perhaps a side effect of Burstein's experiment, Mitchell Tanner soon became mentally unbalanced. 
Warhawk became an assassin used for the C.I.A. in Vietnam. He later went further insane and battled Iron Fist. After Warhawk somewhat regained his sanity he was hired to assassinate Atom-Smasher.
Warhawk was hired by the Hellfire Club to test the X-Men and bug their mansion. He snuck into the school posing as a telephone repair man and quickly took down Moira MacTaggert with a tranquilizer gun. Then he caught Professor X and Phoenix off gaurd. Warhawk took control of the Danger Room, trapping the rest of the X-Men inside. After Nightcrawler teleported out of the Danger Room with Wolverine, Warhawk battled Wolverine one on one. The rest of the X-Men escaped the Danger Room and defeated Warhawk, who was handed over to the police.
Warhawk soon became even more imbalanced and racially bigoted. He took hostages in Chinatown in New York City. He gave Iron Fist a severe beating and assassinated several citizens before being defeated and captured by Luke Cage. 
Warhawk agreed to work for S.H.I.E.L.D. in exchange for a reduced sentence. The had him attack an AIM facility.
Burnstein Process: He possesses various superhuman physical attributes after undergoing an experiment designed to increases his body's cellular regeneration process.
Superhuman Strength: After undergoing the original experiment, his strength was increased to superhuman levels sufficient to lift approximately 3 tons.
Superhuman Stamina: His muscles generate less fatigue toxins than the muscles of ordinary humans, granting him superhuman stamina. He can exert himself at peak capacity for about 24 hours before fatigue begins to affect him.
Superhuman Durability: His skin is as hard as titanium steel and his muscle and bone tissues are considerably denser than the tissues of an ordinary human, granting him much greater resistance to physical injury than an ordinary human. He can withstand conventional handgun fire at a range of 4 feet and cannot be cut by any blade forged of conventional material, although in the event of required surgery his skin can be lacerated by an overpowered medical laser. He can withstand up to one-ton impacts or blasts of 150 pounds of TNT without serious injury, and is highly resistant to extreme temperatures and electrical shocks. His current level of durability enables him to withstand gun fire at point-blank range, and can withstand being lit on fire without suffering any serious or permanent injuries.
Accelerated Healing Factor: Despite his near invulnerability, it is possible for him to be hurt. If injured, he is capable of recovering from mild injuries in 1/3rd the time it would take an ordinary human.