Complex sentence: Even before the outbreak of the Second World War, the Royal Navy had studied the threat in the Mediterranean posed by the Italian fleet, which had its advance base in the new port of Taranto in the "heel" of Italy.
Split into simpler sentences: Even before the outbreak of World War II, the Royal Navy had studied the threat in the Mediterranean posed by the Italian fleet. This had its advance base in the new port of Taranto in the heel of Italy.