This culminated in the Battle of Kōan. The Eastern Route Army arrived at Hakata Bay in Kyushu on June 21, 1281. They proceeded without the larger southern force. Waves of samurai responded and prevented the Mongols from forming a beachhead. The samurai used a harassment tactic by boarding the Yuan ships with small boats at night. They killed many of the Yuan forces in the bay and the samurai left before dawn. This caused the Yuan to retreat to Tsushima. During the next few weeks up to 3000 Yuan were killed in close quarters. On July 16, the first of the Southern force ships arrived. By August 12, the two fleets were ready to attack Japan. However, on August 15 a major typhoon (kamikaze) struck the Tsushima Strait. It lasted two full days and destroyed most of the Yuan fleet. Over 4,000 ships were destroyed in the storm; 80 percent of the Yuan soldiers drowned or were killed by samurai on the beaches. The loss of ships was so great that "a person could walk across from one point of land to another on a mass of wreckage".