The United Kingdom began the world's first nuclear weapons research project, codenamed Tube Alloys, in 1941.
Notes on verification
Confirmed by Wikipedia, a peer-reviewed Nuclear Technology article, and UK Defence Journal, all agreeing on the 1941 start date and the UK's priority in establishing the first nuclear weapons programme.
Sources
- History of nuclear weapons (en.wikipedia.org)
- https://www.tandfonline.com/doi/full/10.1080/00295450.2021.1910004 (tandfonline.com)
- https://ukdefencejournal.org.uk/britain-and-nuclear-weapons-the-second-world-war-to-the-21st-century/ (ukdefencejournal.org.uk)