In March 1941, to support the planned German invasion of the Balkans, Vice Admiral Angelo Iachino's Italian fleet, led by the battleship Vittorio Veneto, sailed to intercept Allied convoys between Egypt and Greece. Warned of the Italian intentions by intelligence from the Government Code and Cypher School at Bletchley Park, Admiral Cunningham took his fleet to sea on 27 March 1941, flying his flag on Warspite. On 28 March the British cruisers encountered the Italian fleet and were forced to turn away by the heavy guns of Vittorio Veneto. To save his cruisers Cunningham ordered an air strike, prompting Iachino to retreat. Subsequent air attacks damaged the battleship and the cruiser Pola, slowing the former and crippling the latter. Vittorio Veneto escaped to the west as dusk fell, but the British pursued through the night, first detecting Pola on radar and then two of her sister ships. Warspite, Valiant, and Barham closed on the unsuspecting Italian ships and, aided by searchlights, destroyed the heavy cruisers Fiume and Zara, and two destroyers at point blank range. Pola was also sunk once her crew had been taken off. Having established by aerial reconnaissance that the rest of the Italian fleet had escaped Warspite returned to Alexandria on 29 March, surviving air attacks without suffering any casualties. The Battle of Cape Matapan had a paralysing effect on the Regia Marina, providing the Royal Navy with an opportunity to tighten its grip on the Mediterranean, as evidenced by the unequal battle near the Kerkennah Islands on 16 April. However, this was not enough and the continuing success of the Afrika Korps in North Africa induced Churchill to order a desperate attack on Tripoli to entirely block the Axis supply route by sinking one of the battleships in the harbour. Cunningham rejected this plan, but on 21 April he sailed with Warspite to bombard the harbour in company with Barham and Valiant, the cruiser Gloucester and several destroyers. The raid was ineffectual, partly because of poor visibility created by dust from an earlier RAF bombing raid, but the fleet returned to Alexandria without suffering any damage. The futility of the mission and the exposure of his battleships led to a tense exchange of letters between Cunningham and Churchill.