Although the advance in Arakan had been halted to release troops and aircraft for the Battle of Imphal, the Americans and Chinese had continued to advance in northern Burma, aided by the Chindits operating against the Japanese lines of communication. In the middle of 1944 the Chinese Expeditionary Force invaded northern Burma from Yunnan. They captured a fortified position at Mount Song. By the time campaigning ceased during the monsoon rains, the NCAC had secured a vital airfield at Myitkyina (August 1944), which eased the problems of air resupply from India to China over "The Hump".