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Commanded by General (later Field Marshal) Sir Bernard Montgomery, 21st Army Group initially controlled all ground forces in Operation Overlord (the United States First Army and British Second Army).
Given the rank of first lieutenant, he served in the Intelligence Branch of the General Staff of the 21st Army Group, commanded by Montgomery.
The 21st Army Group usually consisted of divisions from the British Army and the Canadian Army.
During his repatriation journey, he was allowed to see Montgomery's 21st Army Group preparing for the invasion of Europe, but was told he was in Kent, where Patton's mythical 1st U.S. Army Group was preparing for its invasion.
Thus the engagements around Xiakou in 1934 did not involve Liu's 24th Route Army, but the 21st army of GMD troops garrisoned just across the Sichuan border in Mingshan.