During World War I, the company returned to arms manufacture and greatly expanded its operations. BSA produced rifles and Lewis guns, but also shells, motorcycles and other vehicles for the struggle. In 1920, it bought the assets of a short-lived plane builder Airco.
Post-war, BSA continued to expand the range of metal goods it produced. The BSA Group bought Triumph, making them the largest producer of motorcycles in the world.
The company made automobiles in 1907-1915, 1921-1926, 1932-1939, and 1960. The Daimler nameplate produced cars for BSA from 1910-1915 and 1915-1960. Lanchester cars also became part of the BSA. There were cars bearing the BSA name itself from 1930-1939 . In 1960 Daimler was sold off to Jaguar.
The Group continued to expand and acquire throughout the 1950s but by 1965 competition from Japan and Germany was eroding BSA's market share. By 1972 BSA was so moribund that it was absorbed into Manganese Bronze in a rescue plan initiated by the Department of Industry and many of the acquisitions were separated or sold. The motorcycle business was hard hit - plans to rescue and combine Norton, BSA and Triumph failed in the face of worker resistance and Norton's and BSA's factories were shut-down, while Triumph staggered on to fail four years later. Only the limited NVT Motorcycles survived. Enjoying the rights to the BSA marque, it was bought-out by the management and renamed the BSA Company.
In 1991 BSA Company merged with Andover Norton International Ltd., to form a new BSA Group, largely producing spare parts for existing motorcycles. In December 1994 Colquhoun and Jackson's BSA Group was taken over by a newly formed BSA Regal Group.
The new company, based in Southampton, has a large spares business and has produced a number of limited-edition, retro-styled motorcycles.