Second World War Brigadier Sir Edgar Trevor Williams CB CBE DSO DL (20 November 1912 – 26 June 1995) was a British historian and army military intelligence officer who played a significant role in the Second Battle of El Alamein in the Second World War.

A graduate of Merton College, Oxford, where he obtained a First in modern history in 1934, Williams was commissioned as a second lieutenant in the 1st King's Dragoon Guards in June 1939.

He was recruited to work in military intelligence by Brigadier Francis de Guingand, who later became Montgomery's chief of staff.

Trevor (known to his friends as "Bill") was educated at Tettenhall College, Staffordshire, and then at King Edward VII School in Sheffield after his father was posted there in 1928.

[2] He earned a Master of Arts degree in 1938,[3] and commenced work on his PhD, in which he argued that it was the Treaty of Waitangi that granted Britain sovereignty over New Zealand, and the land was not terra nullius.

[5] The division was sent to Cyrenaica, where, on 24 February 1941, Williams was in command of a troop of C Squadron, 1st King's Dragoon Guards when it was ambushed near El Agheila.

[6] The desert sun affected his already weak eyes, so he was sent to recuperate in Cairo, where he was posted to General Headquarters (GHQ) Middle East Command[1] in which Brigadier Francis de Guingand became Director of Military Intelligence (DMI) in February 1942.

[8] Aware of his lack of expertise, de Guingand selected Williams and James Oliver Ewart to serve on his staff.

The quality of the information coming from Ultra was very high, but over-reliance on it could be very dangerous, both militarily, when Erwin Rommel and the DAK did not act as expected, and professionally, when the DMI was fired for failing to forecast this.

Williams's academic training came to the fore; as an historian, he was accustomed to integrating different sources of information to build up a larger picture.

[8] He would prepare his intelligence summaries in the early hours of the morning by the light of a pressure lamp, wearing a crochet jacket that had once belonged to a German general.

[1] When Lieutenant-General Bernard Montgomery assumed command of the Eighth Army in August 1942, he was impressed with Williams and identified him as the man he wanted to head his intelligence section.

[11] For the Battle of Alam Halfa, Ultra provided information on German intentions that was accurate in every detail except for a two-day delay caused by a shortage of petrol.

It was not the normal practice in the British Army for generals to take staff with them from one assignment to the next, but Montgomery judged correctly that his fame as the victor of the Battle of El Alamein would overcome any objections.

For Operation Overlord, Williams confronted the formidable task of providing estimates months in advance of a volatile situation.

[15] The G-2 (chief intelligence officer) at Supreme Headquarters Allied Expeditionary Force (SHAEF) was Major-General John Whiteley,[16] de Guingand's predecessor as Brigadier General Staff of the Eighth Army.

He also served as a Radcliffe Trustee, as a member of the Nuffield Provincial Hospitals Trust,[1] and as chairman of the Academic Advisory Board which planned Warwick University.