'Plan X' (i) was the Allied plan for the reconquest of the area of Japanese-held northern Burma as far to the south as the line between Lashio and Kalewa (1943/44).
The plan could not be executed fully in 1943 because of the combination of Japanese moves and Allied lack of strength, but was launched in October 1943 with the advance of Lieutenant General Joseph W. Stilwell’s Chinese Army in India forces ('X' Force comprising the New 22nd, 30th and 38th Divisions).
At this time Stilwell was unhappy that no major British offensive was planned for northern and central Burma in 1943, and feared that the Japanese would use this lack of opposition to undertake their own operation, namely an advance into north-eastern India, where the vital supply road to China was being built from the railhead at Ledo. Stilwell therefore decided on a spoiling offensive toward Myitkyina through the Hukawng valley against Lieutenant General Shinichi Tanaka’s 18th Division of Lieutenant General Renya Mutaguchi’s 15th Army.
The progress of Stilwell’s forces through the Hukawng valley was slow as a result of Tanaka’s delaying tactics, and it was only with the arrival of Brigadier General Frank Merrill’s US 5307th Composite Unit (Provisional), otherwise 'Merrill’s Marauders', late in February 1944 that greater rate of progress was made as the Chinese pinned the Japanese while the Americans undertook hooking movements to bring them into the rear of the Japanese blocking units. The Chinese and American forces closed in on Myitkyina during May 1944, but failed to take this vital town against determined Japanese resistance, and it was only with the arrival of Brigadier J. M. Calvert’s Indian 77th Brigade at the end of 'Thursday' (the 2nd Chindit Expedition) that the Allies were able to take Myitkyina as the Japanese evacuated it on 3/4 August.
However, the essentials of 'Plan X' (i) then matured toward the end of 1944 as Stilwell’s last offensive as chief-of-staff to Generalissimo Chiang Kai-shek and US commander in the China-Burma-India Theater. Apart from expelling the Japanese from north Burma, the primary object of this renewed offensive was to reopen land communications with China via the Burma Road from Mandalay once this latter had fallen to the British, though Stilwell planned a link road between the transport heads at Ledo on the Ledo Road and Mongyu on the Burma Road.
The Japanese defence in this region was in the hands of Lieutenant General Kaoru Takeda’s 53rd Division of the 15th Army, now commanded by Lieutenant General Shihachi Katamura, though farther to the south lay Lieutenant General Shinichi Tanaka’s 18th Division and Lieutenant General Yuzo Matsuyama’s 56th Division of Lieutenant General Masaki Honda’s 33rd Army, respectively covering the northern and northern-eastern approaches to Mandalay.
Now available to Stilwell’s Northern Combat Area Command were Brigadier General John P. Willey’s 'Mars' Force, Lieutenant General Sun Li-jen’s Chinese New 1st Army (New 30th and 38th Divisions), and General Liao Yueh-shang’s Chinese New 6th Army (New 14th, 22nd and 50th Divisions), while operating in close touch in southern Yunnan were Lieutenant General Huang Jie’s Chinese 11th Army Group and Lieutenant General Huo Kuizhang’s 20th Army Group of General Wei Li-huang’s Chinese 'Y' Force. Stilwell could also call on Major General F. W. Festing’s British 36th Division provided by Admiral the Lord Louis Mountbatten’s South-East Asia Command via General Sir George Giffard’s 11th Army Group, which became Lieutenant General Sir Oliver Leese’s Allied Land Forces South-East Asia on 12 November 1944.
The Allied advance began on 15 October 1944. In the west the 36th Division moved to the south from Mogaung toward Indaw (captured on 10 December 1944), Myitson (21 February 1945), Mogok (19 March) and Kyaukme (31 March). In the centre the New 6th Army moved on Shwegu (7 November 1944) and across the upper reaches of the Shweli river to Hsipaw (16 March 1945); and in the east the New 1st Army with aid from the 'Y' Force advanced from Myitkyina to Bhamo (15 December 1944) and Lashio (7 March 1945).
The efforts of all these forces were aided by the implementation of 'Extended Capital' to their west during the same period, and linked with each other between 24 and 30 March in the area of Hsipaw, the 'Y' Force having met the New 1st Army at Mongyu on 27 January.
The usual problems had been encountered in the course of this large-scale operation, though added difficulties were the strained relations of Stilwell and Chiang, and the inroads of the Japanese 'Ichi' operation in China proper. The first resulted in Chiang’s insistence that Stilwell be removed, which occurred on 18 October 1944 after the US commander’s promotion to full general: Stilwell was replaced by three US officers, namely Lieutenant General Raymond A. Wheeler as Deputy Supreme Commander, South-East Asia Command, Lieutenant General Albert C. Wedemeyer as chief-of-staff to Chiang (and as such the commander-in-chief of the Chinese forces) and Lieutenant General Daniel I. Sultan as commander of the Northern Combat Area Command. The second problem resulted in Chiang’s removal of two divisions for deployment against 'Ichi' during the second half of December 1944.
Stilwell was replaced just as his offensive was starting, and this led to certain modifications in the original concept of trapping the 33rd Army after the 53rd Division had held Bhamo for a month up to 15 December before fighting its way out to link with the 56th Division just as Chiang removed the best two Chinese divisions in the theatre. In these circumstances Sultan could not hope to fulfil Stilwell’s original intentions and instead concentrated on the opening of the road between Ledo and Mongyu by taking the Japanese-held portion of Burma between Bhamo and Lashio.