For 1899 he had intended to join the US expedition under A Donaldson Smith between the River Nile and Lake Rudolf, but in October 1899, the new Viceroy of India, Lord Curzon appointed Cox Political Agent and consul at Muscat, Oman, inheriting a tense situation between the British, French and Arabs who regarded the area as under their influence.

Cox managed to successfully end French influence in the area; turning the subsidy around, and agreeing that Feisal's son could receive an education in England and visit the Delhi Durbar.

Cox considered peace the priority, in the maintenance of good relations with the Ottomans, who held all the tribal loyalties, whilst prompting India to change policy towards Ibn Saud, the Wahhabi ruler of Nejd and later king of Saudi Arabia, from 1906[citation needed].

On 16 July 1909, after secret negotiation with Cox, assisted by Arnold Wilson, Sheik Khaz'al agreed to a rental agreement for the island including Abadan.

[8] Among his other achievements while at Bushire was the establishment of the state of Kuwait as an autonomous kaza within the Ottoman Empire by the Anglo-Ottoman Convention of 1913, where he improved relations with local ruler, Mubarak, by opening negotiations with Ibn Saud.

[citation needed] The Turks signed a treaty in London on 29 July 1913 concerning Royal Navy patrols in the Persian Gulf littoral, when Cox met then at the Port of Uqair on 15 December 1913.

[9] Captain Shakespear's letter had passed via Riyadh to the Suez Canal in which his secret War-camp negotiations with Ibn Sa'ud, had revealed the latter's deep hatred of the Turks, who brutalised his people and threatened his ancestral rights.

Shortly after his return to India, Sir Percy was sent back to the Persian Gulf as Chief Political Officer with the Indian Expeditionary Force when World War I broke out in August 1914, still with a brief to prevent Turkish entry on the German side.

Sir Percy received immediate authorisation to draft a Treaty of Khufaisa with the Wahhabi ruler with the aim of forming a broader Arab alliance.

The arrival of General Nixon from Simla was "shabby...jobbing" as the military build-up enclosing India's plan to capture Baghdad troubled the veteran political time-servers, morally responsible to humanity and to civilization.

[citation needed] At 1.30 pm on 9 December, Sir Percy and Fry took the formal handover from Head of Vilayet, Vali of Basra, Subhi Bey, ending the Battle of Qurna.

[17][18] On reflection Cox suggested that the 500 departing unit should turn back; but Colonel Gerard Leachman told him the roads being drenched and muddy were impassable.

[21] Nuri's Basra Reform Society were negotiating with Cox when the British appointed the violent and intemperate Sayyid Talib as governor of the province.

During April 1916, Kitchener offered a series of blatant bribes up to £2 million via General Halil "to the people of Kut", disgusted Cox left with Leachman's cavalry brigade sent back to Basra.

During this time he established strong relations with Ibn Saud, the powerful ruler of the Nejd, with whom he had already had dealings while Resident, and when he gained the nickname Kokus.

[32]Acting as High Commissioner, Cox collaborated with former Ottoman officials and tribal, sectarian, and religious leaders and oversaw the creation of a largely Arab provisional government, or "Council of State," with the purpose of seeing the young country through the turbulent period following the revolt.

[33] The satisfactory functioning of this interim government allowed Cox to attend the Cairo Conference, convened by the new Colonial Secretary Winston Churchill in 1921.

On the question of who should rule Iraq, Cox considered the best option to be one of the sons of the Sharif of Mecca, with whom the British had a special relationship during the war due to promises made during the McMahon–Hussein Correspondence.

[32] ...For some time past letters have been passing between Sir Percy and Ibn Saud The Conquest of Hayil by the latter in November makes his frontiers continuous with the Iraq.

[38]For his remaining years as the High Commissioner of Iraq, Cox continued to greatly influence Iraqi government and events in the country, using his power behind the throne to advise and put pressure on Faisal where necessary, including lavish festivities.

[39] In his subsequent eulogy on his friend Gertrude's death, Cox recalled that 'On 20 April 1923 a treaty was signed with Turkey with proviso that "Nothing in this Protocol shall prevent a fresh agreement from being concluded...and negotiations shall be entered into between them before expiration of the above period."

Cox, in effect, became acting King of Iraq and undertook such measures as jailing and transporting those hostile to foreign intervention; silencing opposition parties and media; and even ordering the bombing of tribal insurgents.

"[37] Ahmad Shikara is not as kind, calling Cox's measures "severe and unpopular" and noting that Faisal himself held "strong objections to the High Commissioner's actions.

"[32] Whatever the case, Cox's actions succeeded in preserving the status quo for the British, and Faisal resumed his rule in September after being a reluctant signatory to a twenty-year treaty.

On the one hand the vast country was supposedly acting as a barrier to Russian threats of invasion and Turkish activity in Mesopotamia, and on the other it was a long way from both India and London.

[45][additional citation(s) needed] The remainder of Cox's term as High Commissioner was spent negotiating the Anglo-Iraqi Treaty, years 1921 and 1922, which established "the infant State of Iraq".

In her letters, the famed adventurer, archaeologist, and author Gertrude Bell writes of the effectiveness of Cox's diplomacy: "Ibn Saud is convinced that the future of himself and his country depends on our goodwill and that he will never break with us.

[citation needed] Cox though was in contact with Halil Beg Bedir Khan and members of the Society for the Rise of Kurdistan and argued the Kurdish demands should be considered as well.

In her writing, Bell describes Cox as possessing an "air of fine and simple dignity," praising his "kindness and consideration," and claiming that his disposition towards her amounted to "an absurd indulgence."

She notes the respect that he enjoyed with the peoples of Iraq and when writing about Cox's dealings with Ibn Saud even declares, "It's really amazing that anyone should exercise influence such as his...I don't think that any European in history has made a deeper impression on the Oriental mind."