Training in the United States

On 30 November 1940, Moore became the aide-de-camp to Brigadier General William H. Simpson, the assistant division commander of the 2nd Infantry Division, of which the 29th Infantry Regiment was a part. Simpson would be an officer with whom he would be associated over the next few years.^[6] Moore was promoted to major in the wartime Army of the United States on 31 January 1941. On 12 April he was transferred to the War Department General Staff in Washington, D.C., where he worked in the Budget and Legislative Planning Branch. His rank of major in the Infantry Branch became substantive on 12 June, and he was promoted to colonel on 6 January 1942.^[7]

In March 1942, with the United States now engaged in World War II, Moore became the chief of staff of the 35th Infantry Division at Camp San Luis Obispo, California. This reunited him with Simpson, who was the division commander. The position came with a promotion to colonel in the Army of the United States on 4 March, because the Chief of Staff of the United States Army, General George C. Marshall, had instituted a policy that regular officers taking on the position of chief of staff of an Army National Guard division would have that rank to ensure seniority over the National Guard officers on their staff.^[7][8]

Moore and Simpson were abruptly transferred to the 30th Infantry Division at Fort Jackson, South Carolina, on 22 May 1942. The division had performed poorly during the Carolina Maneuvers in 1941, and Lieutenant Generals Hugh Drum, Ben Lear and Lesley McNair had recommended the relief of the division commander, Major General Henry D. Russell. He was a relative of Senator Richard Russell Jr., so the relief was politically sensitive. The division was in poor shape, having just been reorganized, and a large number of transfers left its strength at less than 3,000 men. Simpson relieved all the regimental commanders. Moore was awarded the Legion of Merit for his work with the division staff.^[7][9] Simpson assumed command of the newly formed XII Corps in August 1942, but Moore remained with the 30th Infantry Division until 10 August 1943, when he rejoined Simpson as chief of staff of XII Corps.^[7][10]

Chief of Staff of Ninth Army

On 13 October 1943, Simpson assumed command of the Fourth United States Army, and Moore went with him as chief of staff.^[11] The Fourth Army headquarters was initially in San Jose, California, and it functioned as a training army. In search of more office space, the headquarters was moved to the Presidio of Monterey, California, on 1 November, and then to Fort Sam Houston, Texas, in January 1944, when it took over the training mission of the Third United States Army, which had moved overseas.^[12] More personnel arrived in early 1944, enabling the Fourth Army to be split into a training army (the Fourth) and a headquarters to be deployed overseas, the Eighth, which was activated on 5 May 1944. Simpson, Moore and most of the staff became part of the Eighth Army headquarters. Moore was part of the advance party of the headquarters that flew to the UK on 11 May, where Simpson met with the commander of the European Theater of Operations, United States Army, General Dwight D. Eisenhower. At Eisenhower's request, the Eighth Army was redesignated the Ninth United States Army to avoid confusion with the British Eighth Army. The main body of Ninth Army headquarters embarked for the UK on the ocean liner RMS Queen Elizabeth on 22 June.^[13]

The Ninth Army headquarters moved to France in August, and it became active as part of Lieutenant General Omar Bradley's 12th Army Group on 5 September when it took over command of the US forces in Brittany from Lieutenant General George S. Patton Jr.'s Third Army.^[14] Simpson's first task was the capture of Brest. There were sufficient artillery pieces in the area but not sufficient ammunition, especially for the heavier pieces. Over a two-week period, 40,000 long tons (41,000 t) of artillery ammunition was brought forward from dumps in Normandy and the UK.^[15] The battle commenced on 8 September and after much hard fighting the city was liberated on 20 September 1944.^[16]

Ninth Army headquarters moved to Arlon, where it opened on 2 October, and two days later the Ninth Army took over the center of the 12th Army Group's front in the Ardennes between the First and Third Armies.^[17] Moore considered moving the headquarters to Luxembourg City but was informed that the communications to the north were not safe. He was therefore surprised when the 12th Army Group headquarters moved there.^[18] On 10 October, Simpson received word that Ninth Army was to take over the northern sector of the 12th Army Group's front adjoining Field Marshal Bernard Montgomery's Anglo-Canadian 21st Army Group.^[19] Foreseeing that one of his armies would be assigned to the 21st Army Group at some point, Bradley preferred that it would be the Ninth rather than the more insular First Army.^[20] Bradley later wrote that: "Unlike the noisy and bumptious Third and the temperamental First, the Ninth remained uncommonly normal."^[21] Reflecting on the decision, Bradley opined that the Ninth Army staff collaborated with the 21st Army Group better than the First Army staff would have.^[21]

During the Battle of the Bulge, both the First and Ninth Armies were transferred to the 21st Army Group after the 12th Army Group's communications with the north were cut. When Montgomery visited Ninth Army headquarters, his reception was genial and professional. Moore established good relationships with the staffs of both Lieutenant General Courtney Hodges's First Army and Lieutenant General Sir Miles Dempsey's British Second Army, but he was not gullible: when six of the Ninth Army's twenty-six truck companies were sent to support the First Army, Moore assigned a staff officer to ensure that they were returned.^[22] He was promoted to major general on 12 April 1945.^[7]

For his service as chief of staff of the Ninth Army, Moore received the Army Distinguished Service Medal, two oak leaf clusters to his Legion of Merit, and the Bronze Star Medal. He also received foreign orders and decorations, including the Order of the Crown with palm and Croix de Guerre from Belgium, the Legion of Honor and Croix de Guerre 1939–1945 from France and the Order of the Red Banner from the Soviet Union. He was made a Grand Officer in the military Order of Orange-Nassau by the Netherlands and an Honorary Commander of the Order of the British Empire by the United Kingdom.^[7][23]