Complex sentence: The Allied forces only made significant progress when they were finally given the tanks and artillery they had long sought when the first large ship, the "Karsik" arrived at Oro Bay 11 December 1942 preceding the regular supply convoys of Operation Lilliput.
Split into simpler sentences: The Allied forces only made significant progress when they were finally given the tanks and artillery they had long sought. The first large supply ship, the "Karsik", arrived at Oro Bay 11 December 1942 preceding the regular supply convoys of Operation Lilliput.