From early in the war British efforts based in Egypt were intended to frustrate the ambitions of Germany’s ally, the Ottoman Turkish Empire, and especially to protect the Suez Canal, the dominions of Australia and New Zealand, and India. Britain negotiated with – among others – Abn bin Saud and Sharif Hussein (various spellings!). Hussein controlled the Hedjaz and the Muslim holy city of Mecca. From early 1916 at least Hussein was working together with the British. His understanding was based on a future independent Arab state, (see elsewhere)
The document attached dated 12th April 1916 hints at the plans made – with the aid of T.E. Lawrence – for operations along the Mecca – Medina – Damascus railway line. (see e.g. A Line in the Sand. James Barr. Simon and Schuster. 2011).