'Rio-1' was a Japanese air attack on the dock and harbour installations of Calcutta in north-eastern India (5 December 1943).
As soon as the monsoon had ended late in 1943, the Japanese became more active in the air over and round the Burma front, and in November 1943 began a series of offensive sweeps. About 130 aircraft were used in attacks on the Allied airfields in north-eastern Assam on 8 and 9 November, but these inflicted little damage.
The next attack had been planned for some time, and was a raid on Calcutta with the object of destroying shipping and harbour installations to delay the Allied preparations for a counter-offensive in Burma. The 7th Air Brigade had rehearsed its effort in Malaya during the middle of November and, toward the end of the month, was reinforced by a number of naval aircraft. As reconnaissance had revealed a large concentration of shipping at Calcutta, the Japanese decided to raid the airfields in the Chittagong area at the end of the month to draw the Allied fighters away from Calcutta, which was to be attacked on 5 December. The Japanese had in fact rightly anticipated the Allied reaction. After attacks on the airfields at Feni and Agartala on the 28 and 29 November, the only Supermarine Spitfire squadron available at that time for the defence of Calcutta was sent to reinforce the two Spitfire squadrons at Chittagong, leaving the defence of Calcutta to two Hawker Hurricane squadrons and one night-fighter squadron. At 09.45 on 5 December some 65 Spitfire and Hurricane fighters responded to a radar warning and lifted off from Chittagong to intercept a Japanese formation approaching over the Bay of Bengal.
The bombers and fighters of the Imperial Japanese army air force’s 7th Air Brigade, reinforced Mitsubishi G4M 'Betty' bombers of the Japanese navy air force’s 705th Kokutai escorted by the Mitsubishi A6M Reisen 'Zero' fighters of the 331st Kokutai, the Japanese launched their attack from Magwe airfield and flew a course that avoided Chittagong. Operating at the very limits of their range the British fighters could not make contact. At Calcutta the Hurricane fighters of Nos 67 and 146 Squadrons, which had been held at instant readiness, had also taken off and at 11.30 intercepted the Japanese aircraft. The 27 bombers were flying at 23,950 ft (7300 m), and were escorted by about 100 fighters both above and below them. Thus the Hurricane fighters were at a tactical disadvantage with numerically superior Japanese fighters.
The bombers attacked the docks and five Hurricane fighters were lost for only one Japanese fighter. Some 30 minutes later a second wave of bombers with fighter escort approached the city. Both Hurricane squadrons were refuelling and re-arming after the first raid, and only six Hurricane night-fighters were immediately available. These intercepted the Japanese formation but were themselves attacked from above. Three of No. 176 Squadron’s Hurricane fighters were shot down and the docks were bombed for a second time. In a final attempt to engage the enemy, two Spitfire squadrons took off from Chittagong in the hope that some of the raiders might be caught as they returned over the Bay of Bengal, but once again the Japanese formation passed out of range.
Although the weight of bombs dropped on Calcutta was small, three merchant vessels and one warship were damaged, 15 barges were set on fire, and hits were obtained on nine dock sheds, two of which were gutted by fire, and there were about 500 civilian casualties.
As had been the case in previous raids on Calcutta, many thousands fled the city and only one-tenth of the normal labour force remained at work at the docks.
A false alert on the following day further diminished civilian morale in Calcutta. As a temporary measure a few Spitfire fighters were brought back from Chittagong, partly to help in restoring morale, but the raids, which had little effect on military preparations, were not repeated.