Complex sentence: When the dark clouds of war were gathering in the East in 1941, the staff and students had to practise Air Raid Drills and true to expectation, the first bombs were dropped in Singapore in December 1941.
Split into simpler sentences: When the dark clouds of war were gathering in the East in 1941, the staff and students practised air raid drills. The first bombs were dropped in Singapore in December 1941.