Complex sentence: The Battle of Singapore, also known as the Fall of Singapore, was fought in the South-East Asian theatre of the Second World War when the Empire of Japan invaded the British stronghold of Singapore--nicknamed the "Gibraltar of the East".
Split into simpler sentences: The Battle of Singapore, also known as the Fall of Singapore, was fought in the South-East Asian theatre of the Second World War when the Empire of Japan invaded the British stronghold of Singapore. Singapore was the major British military base in South-East Asia and nicknamed the "Gibraltar of the East".