This was a US unrealised plan for the forces of Lieutenant General Walter C. Krueger’s 6th Army to land at Vigan on the north-west coast of Luzon island in the Philippine islands group to speed the US advance to the west of the Cordillera Central massif in the north of Luzon, held by the ‘Shobo’ Group of General Tomoyuki Yamashita’s 14th Area Army (January/February 1945).
Commanded directly by Yamashita and based on Lieutenant General Yasuyuki Okamoto’s 10th Division, Lieutenant General Yoshiharu Ozaki’s 19th Division, Lieutenant General Fukutaro Nishiyama’s 23rd Division and Lieutenant General Yoshitake Muraoka’s 103rd Divisions, together with Lieutenant General Yoshiharu Iwanaka’s 2nd Armoured Division and Major General Binzo Sato’s 58th Independent Mixed Brigade, the ‘Shobo’ Group at the end of January 1945 numbered some 110,000 men, and because elements of his army were still heavily involved in the centre and south-east of Luzon, Krueger could deploy only Major General Charles L. Mullins’s 25th Division, Major General William H. Gill’s 32nd Division and Major General Percy W. Clarkson’s 33rd Division of Lieutenant General Innis P. Swift’s I Corps, supported by part of Major General Robert S. Beightler’s 37th Division, against the ‘Shobo’ Group.
By 26 April the 33rd Division, with support from the 37th Division, had taken Yamashita’s erstwhile headquarters at Baguio, but the whole of May and June 1945 passed before the I Corps could break through the Balete Pass to push on to Bambang and the upper reaches of the Cagayan river valley between the Cordillera Central massif and the Sierra Madre mountains. The 37th Division pushed down the valley, taking Ilagan on 19 June and Tuguegarao on 25 June before linking on 26 June with the 511th Parachute Infantry, which had dropped at Aparri in the extreme north of Luzon on 23 June. Other units had meanwhile advanced up the west coast of northern Luzon, taking San Fernando on 26 March and Vigan on 19 April before advancing along the northern coast to Aparri. By the end of June Yamashita had been reduced to some 65,000 men, but these forces now held out for the rest of the war in two major pockets, one under Yamashita in the mountains to the south of Bontoc and the other in the Sierra Madre mountains on the west coast of northern Luzon.
There would have been little point in the I Corps attempting to reduce these mountain garrisons, for US casualties would have been enormous for little or no strategic benefit, but the presence of the two Japanese pockets tied down the 25th, 32nd, 33rd and 37th Divisions until the formalisation of Japan’s surrender in September 1945.