Opening phases.
Indian IV Corps in Imphal was commanded by Lieutenant-General Geoffrey Scoones, and was in turn part of the British Fourteenth Army under Lieutenant-General William Slim. When they received intelligence that a major Japanese offensive was impending, Slim and Scoones planned to withdraw into the Imphal plain and force the Japanese to fight with their logistics stretched beyond the limit. However, they misjudged the date on which the Japanese were to attack, and the strength they would use against some objectives. The Japanese launched their troops across the Chindwin River on 8 March 1944. Scoones only gave his forward divisions orders to withdraw to Imphal on 13 March.
The Indian 20th Division under Major-General Douglas Gracey withdrew without difficulty, mainly because two of Yamamoto's battalions from the Japanese 15th Division were delayed at Indaw in northern Burma by the Chindits and were unable to intervene.
The Indian 17th Division under Major-General Cowan was cut off by the Japanese 33rd Division. The Japanese 215 Regiment captured a supply dump at Milestone 109, twenty miles behind Cowan's leading outposts. The Japanese 214 Regiment seized Tongzang and a ridge named Tuitum Saddle across the only road, a few miles behind the Indian 17th Division's position.
At Tuitum Saddle, 214 Regiment were unable to dig in properly before they were hit by the Indian 48th Brigade on 18 March. The Japanese suffered heavy casualties and were forced away from the road. Fighting around Milestone 109 was even more severe, but Cowan had taken steps to secure the most vulnerable point in the rear of his division, the bridge over the Manipur River. 17th Division crossed safely, demolishing the bridge behind them, and recovered the depot on 25 March. They were forced to abandon large amounts of supplies, but removed most of the vehicles, food and ammunition. The Japanese were left only such items as clothing and blankets.
Scoones had nevertheless been forced to send the bulk of his only reserve, Indian 23rd Infantry Division under Major-General Ouvry Roberts, to the aid of 17th Division. The two divisions, now supplied by parachute drops from Allied aircraft, made their way back to the Imphal plain, which they reached on 4 April.
Meanwhile, Imphal had been left vulnerable to the Japanese 15th Division. The only force left covering the base, Indian 50 Parachute Brigade, was roughly handled at Sangshak by a regiment from the Japanese 31st Division on its way to Kohima. The 31st Division had also blocked the main road south of Kohima by the start of April, cutting off IV Corps. However, an earlier diversionary attack launched by Japanese 55th Division in Arakan had already failed. Slim was able to move the battle-hardened Indian 5th Infantry Division, including all its artillery and transport, by air from Arakan to the Central Front. The move was completed in only eleven days. Two of its brigades went to Imphal, and their leading troops were in action on 3 April.
Stalemate.
From the beginning of April, the Japanese attacked the Imphal plain from several directions:
33rd Division attacked from the south at Bishenpur, where they cut a secondary track from Silchar into the plain. Yanagida, its commander, was already pessimistic and depressed by the failure to trap the Indian 17th Division. He had also been rattled by a garbled radio message which suggested that one of his regiments had been destroyed at Milestone 109. He therefore advanced cautiously. By doing so, he may have lost a chance to gain success while the Indian 17th Infantry Division was resting after its retreat and Bishenpur was held only by Indian 32 Brigade (from 20th Division). Mutaguchi removed him from command.
Yamamoto Force attacked the Shenam Saddle on the main road from Tamu into Imphal. The Shenam Saddle was ideal defensive terrain. Despite using heavy artillery and tanks, Yamamoto could not break through Indian 20th Division's well-sited defences. Detachments from two brigades of the Indian National Army fought on the Japanese side in this sector. The initial enthusiasm of most INA soldiers soon waned, and they deserted and straggled in increasing numbers.
15th Division encircled Imphal from the north. Its 60 Regiment captured a British supply dump at Kangpokpi (also known as "Mission" from a church there) on the main Imphal-Dimapur road, but once again, the depot had already been emptied of food and ammunition. 51 Regiment seized the vital Nunshigum Ridge, which overlooked the main airstrip at Imphal. This was a major threat to IV Corps, and on 13 April the Indian 5th Division counter-attacked, supported by massed artillery and the M3 Lee tanks of the 3rd Carabiniers. The Japanese regiment had no anti-tank weapons, and their troops were driven from the ridge with heavy casualties.
Allied counter-attacks.
By 1st May, all Japanese attacks had come to a halt. Slim and Scoones now began a counter-offensive against the Japanese 15th Division. This division was the weakest of the Japanese formations, and success against it would break the siege. Progress was slow. The monsoon had broken, making movement very difficult. Also, IV Corps was suffering some shortages. Although rations and reinforcements were delivered to Imphal by air, artillery ammunition was by now rationed. The steep ridges held by the Japanese were almost impregnable.
However, the Japanese were at the end of their endurance. Neither the Japanese 31st Division which was fighting at Kohima, nor 15th Division, had received adequate supplies since the offensive began, and their troops were starving. This allowed Indian XXXIII Corps to drive the Japanese from Kohima at the end of May, and advance south. The troops of Japanese 15th Division were forced to abandon their defensive positions to forage for supplies in local villages. The leading troops of IV Corps and XXXIII Corps met at Milestone 109 on the Dimapur-Imphal road on 22 June, and the siege was raised.
Royal and Allied Air Force Operations at Imphal.
By mid-1944, the Allied air forces controlled the skies over Burma. The last major effort by the Japanese Army Air Force had been over the Arakan and Imphal in February and March, when they had suffered severe losses.
IV Corps enjoyed close air support from fighter-bombers and dive bombers. Allied fighter bombers and medium bombers shot up and bombed enemy concentrations, supply dumps, transport, roads and bridges all the way to the Chindwin river. The monsoon in no way diminished their activity. The Third Tactical Air Force (TAF) increased their sortie rate to 24,000 sorties during the worst four months of the monsoon, nearly six times the figure of the previous year's record.
The Allies could fly men, equipment and supplies into the airstrips at Imphal so although cut off by land, the town was not without a lifeline. Allied aircraft could also parachute ammunition, rations and even drinking water to surrounded units.
RAF Third Tactical Air Force.
The RAF Third Tactical Air Force (Third TAF), which was formed in south Asia in December 1943, was one of three tactical air forces formed by the Royal Air Force during World War II. Third TAF was formed shortly after the establishment of South East Asia Command to provide close air support to the Fourteenth Army.
It was first formed on 19 December 1943 designated the Tactical Air Force (Burma) and renamed as the Third TAF on 28 December 1943. Along with parts of the USAAF Tenth Air Force, it was subordinate to Joint Allied Eastern Air Command which was also formed in December 1943.
As the Air Force was formed, it was felt that at last British forces could go over to the offensive against the Japanese in the Burma Campaign. A start was made towards establishing a general offensive in Arakan in early 1943, but this was forestalled by a Japanese offensive. The Japanese were decisively beaten, but they shifted the focus of their attack to central Burma. Third TAF gave sterling service to Fourteenth Army during the Battle of Kohima and the Battle of Imphal, strafing and bombing the besieging Japanese troops, often at very low level.
After the defeat of the Japanese by Indian IV and XXXIII Corps in Assam, the monsoon intervened before many counterattacks could take place. After the enforced period of reduced operations, the Third TAF supported the advance of Fourteenth Army against the Japanese forces. However, command arrangement changes at the end of 1944 cutting short the life of the Third TAF. It was redesignated HQ RAF Bengal and Burma on 4 December 1944.
USAAF Air Commando Group.
The British were substantially assisted in supply and Chindit operation by the US Army Air Force in Burma. Substantial numbers of C47's and gliders were used used during Operation Thursday and in the resupply of Imphal. US P51's and P38's also carried out strikes against Janaese airfields and communications as well as supporting their own drive south from the Chinese border at Ledo.

US Army helicopter Burma mid 1944