Trench warfare
Trench warfare prevailed on the Western Front from late 1914 until the Germans launched their Spring Offensive on 21 March 1918. After the buildup of forces in 1915, the Western Front became a stalemated struggle between equals, to be decided by attrition. Frontal assaults, and their associated casualties, became inevitable because the continuous trench lines had no open flanks. Casualties of the defenders matched those of the attackers, as vast reserves were expended in costly counter-attacks or exposed to the attacker's massed artillery. There were periods in which rigid trench warfare broke down, such as during the Battle of the Somme, but the lines never moved very far. The war would be won by the side that was able to commit the last reserves to the Western Front.
Gallipoli Campaign
By late 1914 the war on the Western Front had become a stalemate; the Franco-British counter-offensive of the First Battle of the Marne had ended and the British had suffered many casualties in the First Battle of Ypres in Flanders. The war of manoeuvre ended and trench warfare began. The German Empire and Austria-Hungary closed the overland trade routes between Britain and France in the west and Russia in the east. The White Sea in the arctic north and the Sea of Okhotsk in the Far East were icebound in winter and distant from the Eastern Front, the Baltic Sea was blockaded by the "Kaiserliche Marine" (Imperial German Navy) and the entrance to the Black Sea through the Dardanelles was controlled by the Ottoman Empire. While the empire remained neutral supplies could still be sent to Russia through the Dardanelles but prior to the Ottoman entry into the war the straits had been closed and in November the Turks began to mine the waterway.