The Kamakura shogunate anticipated a second invasion so they constructed walls and fortresses along the shore and gathered forces to defend it. The second Mongol invasion was the largest naval invasion in history until D-Day. In the spring of 1281, Kublai Khan sent two separate forces. An impressive 900 ships containing 40,000 Yuan troops set out from Masan, Korea, while an even larger force of 100,000 sailed from southern China in 3,500 ships. The Mongols planned an overwhelming coordinated attack by the combined imperial Yuan fleets. The Chinese fleet of the Yuan was delayed by difficulties in provisioning and manning their large number of ships.