The Allies do not appear to have entered on the war with any very definite notions. Britain and France formed an alliance together, and then allied themselves with the Sultan. In defending the Sultan, they were to defend a fundamental principle of European policy in the concrete, and they were to take no advantage to themselves by the act. But their earlier notions were limited even from the defensive point of view. They determined to secure a line of retreat for their ships, and a base of operations from which, in the event of the Turkish army being driven over the Balkan, they could effectively defend Constantinople. At this time there was existent an exaggerated dread of Russian power. The Czar was so strong, the Sultan so weak, so men thought, that it was deemed possible the Russians might force both the Danube and the Balkan by the rapid marches of an overwhelming force, and thus confront the Sultan in his capital. To provide against this, and also to cover their weakness, the Allies determined to land their troops at Gallipoli at the mouth of the Dardanelles. Therefore, as the allied troops began to arrive in March and April, they were employed in throwing up entrenchments, known as the lines of Boulair, extending from the Gulf of Saros to the Sea of Marmora. It was in the camps near Gallipoli that the whole of the French and part of the British army were organised for active service; but while they were assembling there, the Turks were fighting so manfully on the Danube, and so effectually thwarting Russia, that the lines became useless, and the Allies found it needful to take post on the northern instead of the southern slopes of the Balkans.