Complex sentence: Churchill had become one of the most prominent members of the Government outside the Cabinet, and when Campbell-Bannerman was succeeded by Herbert Henry Asquith in 1908, he was promoted to the Cabinet as President of the Board of Trade.
Split into simpler sentences: Churchill had become one of the most prominent members of the Government outside the Cabinet. When Campbell-Bannerman was succeeded by Herbert Henry Asquith in 1908, he was promoted to the Cabinet as President of the Board of Trade.