The determination of various European powers to establish a presence in the Middle East elicited an equally firm determination in others to thwart such goals. The most important participants in the drama as far as Yemen was concerned were the British, who took over Aden in 1839, and the Ottoman Empire, which at mid-century moved back into (North) Yemen, from which it had been driven two centuries earlier. The interests and activities of these two powers in the Red Sea basin and Yemen were substantially intensified by the opening of the Suez Canal and the reemergence of the Red Sea route as the preferred passage between Europe and the Far East. As the Ottomans expanded eastward (inland toward San'a') from the coast, the British expanded north and east from Aden, more in the interest of protecting Aden's hinterland from the territorial pretensions and military incursions of the imams than of adding the various entities there to the empire. By the early 20th century, the confrontations between the British and the Ottomans required a border agreement; in 1904 a border commission surveyed the area, and a treaty established the frontier between (Ottoman) Yemen and the British possessions in (South) Yemen. Later, of course, both Yemens considered the treaty an egregious instance of non-Arab interference in Arab affairs.