Complex sentence: Construction of both ships began in 1920, however, the 1922 Washington Naval Treaty intervened; this mandated the cancellation of all naval ships under construction, including the two "Tosa"-class battleships; See: construction stopped on both ships on 5 February 1922.
Split into simpler sentences: Construction of both ships began in 1920, but the 1922 Washington Naval Treaty intervened, mandating the cancellation of all naval ships under construction. Work stopped on the two "Tosa"-class battleships; See: on 5 February 1922.