Complex sentence: MI5 by the Directorate of Military Intelligence (DoMI) under Captain (later Major-General) Vernon Kell; overseas intelligence gathering began in 1912 by MI6 under Commander (later Captain) Mansfield Smith-Cumming.
Split into simpler sentences: MI5 by the Directorate of Military Intelligence (DoMI) under Captain (later Major-General) Vernon Kell. Overseas intelligence gathering began in 1912 by MI6 under Commander (later Captain) Mansfield Smith-Cumming.