As a teenager, Brian Braddock was fascinated by the World War II super-soldier Captain America, even hanging a poster of the hero in his room at Fettes College, his Edinburgh boarding school. Inheriting the scientific aptitude of his bio-engineer father, Sir James Braddock, Brian eventually joined him at the Dome in Brussels, working for the European Defense Initiative's super-soldier program. Each member country provided a team that worked on its own designs, with the elder Braddock overseeing the project as a whole. Brian worked on designing an exo-suit which could enhance the wearer's abilities. Allegedly Thorlief Golman stole the Norwegian team's successful prototypes to become Thor. Despite this setback, the project continued working to create a European Union equivalent of the Ultimates taskforce forming in the United States.
Personal tragedy hit the Braddocks when Brian's sister Betsy, a Psi agent of S.T.R.I.K.E. (the British branch of S.H.I.E.L.D.), was possessed by the rogue mutant David Xavier and slain by the X-Men, though the family soon learned she had survived by taking over the body of a long-term coma victim. Soon after this, Brian perfected his armor and took the codename Captain Britain. With twelve other candidates from the E.U. member states in various states of readiness, the E.D.I. prepared to go public. One of Brian's earliest missions prior to this unveiling was to raise a downed submarine, assisted by two fellow E.D.I. agents and the Ultimates' Iron Man.
Weeks later, S.H.I.E.L.D. decided Thor had become a dangerous liability who needed to be apprehended. The four combat-ready E.D.I. super-soldiers - Captain Britain, Captain Spain (Carlos Fraile), Captain France (Hugo Etherlinck) and Captain Italy (Umberto Landi) - accompanied the Ultimates to Norway to confront Thor. After a protracted melee, the European super-soldiers helped their allies strip Thor of his belt, the source of his powers, and placed Thor in custody. The entire E.D.I. team joined the Ultimates soon after, invading and disarming a Middle Eastern nation.