On the central of the three fronts the Turks were perhaps in a specially strong position, for they had an established secondary base in Mesopotamia, with its famous capital Baghdad, to which extended, though with one or two gaps, the great strategical railway from Constantinople. Beyond Baghdad they held the lines of the Euphrates and Tigris rivers, and could push eastward into Persia by the highroad which from time immemorial has seen the march of conquering armies eastwards and westwards,--the armies of the Persians for Marathon and Platæa--the armies of Alexander the Great on their way to India--and numberless others before and since. But what German and Turk alike failed to understand, or at all events to appreciate at its full value, was the sea-power of England. Sea-power had in old days given Rome the mastery over Carthage, and in later times it had enabled England to wear out Napoleon. It was to be the deciding factor now in the overthrow of the Turkish Empire, and with the Turkish Empire, of the great German scheme of world domination. For recognising at once the great danger to India of letting the Turks push forward into Persia, and possibly into Afghanistan, recognising also the value of the Persian oil-fields and other British interests in that ancient country, Great Britain had determined not to await Turkish and German aggression on its Indian frontier, but to meet the threat with a bold offensive on Turkish soil. Directly it became certain that Turkey had thrown in her lot with the Central Powers, in the autumn of 1914, an expeditionary force sailed from India for the Persian Gulf, and seized the mouth of the Shat-el-Arab, by which the Tigris and Euphrates pour into the sea. The objects of this expedition were at first limited. The protection of the oil-wells, of such importance to our Navy, and the blocking of the German strategical railway through Baghdad, were all that was immediately contemplated. But the comparatively easy success of the Indian force, mainly composed of native Indian soldiery, in defeating the Turkish troops near the coast, encouraged the British commanders to push on up the rivers into Mesopotamia. In 1915, a year after the outbreak of war, a force under General Townshend had taken Kut-el-Amara, three hundred miles from the sea, and the attack on the Dardanelles being on the point of open failure, it was decided that as a counterblast to this failure Great Britain should strike a great blow in the East by marching to Baghdad and conquering all Turkish Arabia. [Illustration: MESOPOTAMIA]