The clause that would be argued over for the next hundred years did not originate with the British at all. In October 1915, Sir Henry McMahon, His Majesty's High Commissioner at Cairo, was working through his reply to the fourth letter he had received from Hussein bin Ali, Sharif of Mecca and Emir of the Hejaz, who had proposed an independent Arab state stretching from the Taurus mountains to the Egyptian frontier in exchange for launching a revolt against the Ottomans. McMahon needed a way to carve French claims on the Syrian coast out of the deal without saying so outright, and according to declassified British cabinet papers, the specific wording came from an Arab intermediary named al-Faroqi, a member of the nationalist al-Ahd party, who suggested the British could exclude "portions of Syria lying to the west of the districts of Damascus, Homs, Hama, and Aleppo" as not being "purely Arab." London's Foreign Secretary, Lord Grey, approved the language, and McMahon sent it back to Hussein on October 24, 1915, as though the ambiguity were a simple geographic fact rather than a phrase built by committee to paper over an argument nobody wanted to have directly.
Hussein read the letter as confirming Palestine's inclusion in the promised Arab state, since Palestine sat south, not west, of the named line. The letters kept coming through March 1916 — ten in total, most of them in Arabic, several with English translations that would later be shown to diverge from each other in small but consequential ways — and Hussein launched the Arab Revolt against Ottoman rule that June, staking his family's future and roughly seventy thousand fighters on his reading of a sentence a British official had lifted from an Arab officer trying to solve a different problem.
Nine days before that revolt began, on May 16, 1916, the British diplomat Mark Sykes and his French counterpart François Georges-Picot finalized a secret agreement in London that had nothing to do with reconciling itself to McMahon's letters and everything to do with dividing the same territory into zones of direct and indirect control between the two empires. Britain took the area around Baghdad and southern Mesopotamia outright and secured influence over a strip running from the Mediterranean to Persia that included Palestine; France took Lebanon, coastal Syria, and a sphere of influence stretching inland to Mosul. Hussein was not shown the agreement and did not learn of its terms until the Bolsheviks, after seizing power in Russia the following year, published the czarist archives and handed the document to the press specifically to embarrass the Allied governments that had signed it.
The third promise followed on November 2, 1917, and this one was never meant to be secret at all. Foreign Secretary Arthur Balfour wrote to Lord Rothschild that His Majesty's Government viewed with favour the establishment in Palestine of a national home for the Jewish people, a sentence that had gone through several internal drafts and answered years of Zionist lobbying, wartime calculations about American and Russian Jewish opinion, and Britain's own interest in a friendly power astride the approach to the Suez Canal. It ran sixty-seven words, including a clause about not prejudicing the civil and religious rights of the non-Jewish communities already living there — the same Palestine that sat, on Hussein's reading of the letter he'd received two years earlier, inside the borders of the Arab state he had just gone to war to win.
By the time the Mandate for Palestine formally incorporated the Balfour Declaration in 1922, none of the three documents had been reconciled, and none of the men who negotiated them paid the full cost of that failure personally. Hussein did. He never accepted his exclusion from the settlement that followed, refused to sign the Treaty of Versailles or ratify the mandates carving up the region, and abdicated in October 1924 as the rival Ibn Saud's forces took Mecca and Medina, a conquest Britain declined to prevent. He was living in Jeddah when the port itself fell under siege the following year, and in December 1925 a Royal Navy vessel carried him into exile on Cyprus — the same power that had excluded him from the settlement now granting him the only asylum on offer. His wife Adila Khanum died there in July 1929. Hussein's own health failed not long after, and he was moved to Amman, in the kingdom his son Abdullah now ruled, where he died on June 4, 1931, having spent his last years watching the territory he believed a British high commissioner had promised him get parceled out by the same government, on the strength of two agreements he had never been shown and one letter he had never been allowed to misunderstand alone.