on Guadalcanal now operational, convoys of slow transports couldn't go near the island without great risk. Consequently, until the airfield was suppressed, Japanese reinforcements were most-often delivered by inefficient destroyer runs to the island by night. By early September, destroyers had delivered 6,200 troops for an attack on the American perimeter, but the Japanese had underestimated American forces on the island, assuming there were only 2,000 Marines were on the island: the actual number was about 20,000. The attacks launched by the Japanese on the nights of September 12–14 therefore failed. The Japanese did better in the struggle for control of the waters around Guadalcanal. On September 15, the submarine sank the carrier , leaving only a single American carrier, the , active in the Pacific. But although the Japanese possessed up to six operational carriers during the same period, they failed to capitalize on the opportunity. By now the Japanese realized that Guadalcanal was a pivotal contest, so for the next offensive an entire Army division was assigned, with plans to get it to Guadalcanal by mid-October for an offensive beginning October 20. To support this effort, the Combined Fleet stepped up night runs by destroyers and high-speed seaplane carriers (which carried the soldiers' heavy equipment) to Guadalcanal, and a transport convoy was assembled. Aerial bombardment of Henderson Field would be intensified, and surface warships would bombard the airfield. Admiral Yamamoto now defined the Combined Fleet's primary mission as supporting the recapture of the island, with destruction of the US Pacific Fleet a secondary objective. On the night of October 13–14, the battleships and bombarded the airfield on Guadalcanal with 918 rounds, destroying over 40 aircraft and putting the airfield temporarily out of commission. (Mark Stille notes that despite Japan's prewar preoccupation with a titanic clash of battleships, this was the most successful Japanese battleship operation of the war.) The convoy arrived during the night of October 14–15, preceded by two heavy cruisers which shelled the airfield. Aircraft from two carriers flew cover over the transports. American aircraft succeeded in sinking three of the six transports, but not before a total of 4,500 men had landed along with two-thirds of their supplies and equipment. The Japanese kept up the pressure with another cruiser bombardment on the night of October 15–16 and more reinforcement destroyer runs. After several delays, the Japanese began the offensive on October 24. The main attack