Brian and his sister Jacqueline where children of a wealthy British lord at the time of the second World War. Their quiet life in the English countryside was cut short by Prime Minister Chamberlain's appeasement of Hitler in 1938, and Brian violently disagreed with his father's isolationist ideals. Brian left home and, with his friend Roger Aubrey, traveled to Germany. Their appearance scored an important propaganda victory for the Nazis, but when the two men, disenchanted with Germany, attempted to leave, they were imprisoned.

Brian shared a cell with a German scientist who tried to recreate the super-soldier formula that created the hero Captain America but would not let his experiment fall into Nazi hands. Instead, he gave the only sample to Brian, who drank the solution and found his strength, speed, stamina, and reflexes enhanced to peak-human levels. Brian escaped prison and became a costumed Nazi-fighter within Germany, calling himself the Destroyer.

As the Destroyer, Brian fought for the Allied cause within Germany. On one adventure, he rescued the hero team Invaders and his father from the Nazi villains Master Man and Warrior Woman. He was also reunited with Roger Aubrey, who, as Dyna-Mite, had been brainwashed and returned to England, before being freed by the Invaders. Reconciling with his father, Brian learned of his father's costumed identity of Union Jack during the first World War. Brian then took up the mantle of Union Jack at his father's request, returning to Falsworth Manor in England and eventually joining the Invaders. (Roger would then take on the mantle of the Destroyer.)

During a battle with Thor, the Norse god of thunder who was duped into aiding the Nazi cause, Union Jack was mortally wounded by Thor's lightning from his mystical hammer. Coming to his senses, Thor healed Union Jack by drawing the electricity back into Mjolnir, but the process left Union Jack with electricity-generating powers. His newfound powers enabled Falsworth to recover from a period self-doubt, and he continued to serve as an Invader until the war's end.

Following the war's end, Union Jack was among the Invaders who became the All-Winners Squad. He also joined his sister and the Destroyer in hunting Nazi war criminals. By 1951, they reorganized the V-Battalion with other heroes of the time, becoming the organization's first ruling Penance Council.

Union Jack died in a tragic car accident, his romantic relationship with the Destroyer a secret from his father.