[5] Shortly after the outbreak of World War I in August 1914, Willcox's battalion, serving as part of the 18th Brigade of the 6th Division, was sent to the Western Front, landing at St-Nazaire, France on 11 September.

[15] On 16 December 1917, he was appointed a General staff Officer Grade 2 (GSO2) with the temporary rank of major and was also awarded the Distinguished Service Order (DSO) around this time.

[19][1] He served initially in Home Forces, then at Aldershot, in Mesopotamia and Iraq, then at Southern Command and finally attended the Staff College, Camberley from 1925 to 1926.

[3] Among his fellow students there included Ronald Scobie, Philip Whitcombe, Francis Tuker, Roland Le Fanu, Frank Messervy, William Morgan, Raymond Briggs, Gordon Grimsdale, Ralph Deedes, Alan Pigott, John Swayne, Brocas Burrows, Douglas McConnel, Leonard Hawes, Eric Harrison, William Oxley and Langley Browning, all of whom would become general officers.

[27] On 29 June 1937, his permanent rank was advanced to colonel (with seniority backdated to 1 July 1933) and, on the same date, was made an instructor at the Staff College, Quetta in India.

[1] A first-line Territorial Army (TA) formation then stationed in Yorkshire, the division – comprising George Sutton's 125th, Eric Miles's 126th and John Smyth's 127th Infantry Brigades, plus divisional troops – had recently fought in France and subsequently participated in the Dunkirk evacuation, suffering severe losses in both manpower and equipment and needed to be brought up to strength to face a potential German invasion of Britain which, in the aftermath of the French surrender, was considered highly likely.

However, in October 1941, General Sir Alan Brooke, the Commander-in-Chief, Home Forces, who had been one of "Ulysses" Willcox's instructors at the Staff College, Camberley in the mid-1920s and soon to become Chief of the Imperial General Staff (CIGS), believed Willcox to be out of his depth in corps command, claiming him, along with Ridley Pakenham-Walsh, the GOC of IX Corps stationed further north, to be "quite incapable of handling the forces under orders!

[2] After a military career spanning three decades, and with the war over, Willcox, after being knighted, retired from the army in August 1946, retaining the honorary rank of lieutenant general.