Some submarines were made available for supply runs to Lae but they did not have the capacity to support the troops there by themselves. An operation was carried out on 29 March in which four destroyers delivered 800 troops to Finschhafen but the growing threat from Allied aircraft led to the development of routes along the coast of New Guinea from Madang to Finschhafen and along the north and south coasts of New Britain to Finschhafen, thence to Lae using Army landing craft. It was by this means that the remainder of the 51st Division finally made the trip to Lae in May. The necessity of delivering troops and supplies to the front in this manner caused immense difficulties for the Japanese in their attempts to halt further Allied advances. After the war, Japanese officers at Rabaul estimated that around 20,000 troops were lost in transit to New Guinea from Rabaul, a significant factor in Japan's ultimate defeat in the New Guinea campaign.