The defeat of Allied troops at the Battle of Jitra by Japanese forces, supported by tanks moving south from Thailand on 11 December 1941 and the rapid advance of the Japanese inland from their Kota Bharu beachhead on the north-east coast of Malaya overwhelmed the northern defences. Without any real naval presence, the British were unable to challenge Japanese naval operations off the Malayan coast, which proved invaluable to the invaders. With virtually no remaining Allied planes, the Japanese also had mastery of the skies, leaving the Allied ground troops and civilian population exposed to air attack.