Complex sentence: The invasion started a lesser-known chapter of World War II when Commonwealth forces--mainly British, Indian and Canadian--began the futile defence of British Hong Kong which had been deemed militarily undefendable by the War Office.
Split into simpler sentences: The invasion started a lesser-known chapter of World War II when Allied forces--mainly British, Indian and Canadian--began the futile defence of British Hong Kong. Records now show that the territory had been deemed militarily undefendable by the War Office.