Airplane construction and use developed more rapidly than any other feature in the war. After the observation machines, came the battle-planes, whose first purpose was to clear the way and protect the observation planes. Later, heavy machines for bombing expeditions were constructed; and squadrons of airplanes now took part in every battle, preceding the attacking party, and firing with machine-guns and bombs upon the enemy's trenches or his massed troops back of the line. [Illustration: Map] THE RUSSIANS INVADE TURKEY IN ASIA.--In the early months of 1916 Russian troops met with success in an offensive in the part of Turkey south of the Caucasus. This territory, known as Arme´nia, is inhabited by a Christian population who for many years had been the victims of Turkish persecutions; half a million were cruelly exterminated after Turkey allied herself with Germany in 1914. The Russians advanced steadily, inflicting serious defeats upon the Turkish forces. In February they took possession of Erz´erum, a strongly fortified city of Armenia. The capture of this point was of importance because it was a step in the plan for coöperation with the British armies which were pushing their way north from the region of the Persian Gulf. It had the further important result of interrupting Turkish plans for an invasion of Egypt by way of the Isthmus of Suez, as Turkey was compelled to concentrate her power for the defense of her own territory. In April, Treb´izond, the most important city on the Turkish shore of the Black Sea, surrendered to the invading Russian army. The Russians, supported by fleets along the coast, had made the defense of the city impossible. The fall of Trebizond was a very serious blow to the power of Turkey in Asia Minor. THE CAMPAIGN IN MESOPOTAMIA.--Part of the Allied plan in the east was for the junction of Russian armies operating from the region of the Caucasus with British troops from the land around the Persian Gulf. While the Russians, as we have seen, were making a noteworthy success of their part of this program, the British had not been so fortunate. Their plan was to take possession of Mesopotamia, the valley of the Tigris-Euphrates, and occupy its capital, the famous city of Bagdad. General Townshend with an insufficient force had begun his march up the Tigris River the year before and in March, 1915, had occupied the stronghold of Kut-el-Ama´ra, about 100 miles below Bagdad. Here later he was besieged by a Turkish army. A Russian army on the way from Erzerum and an English relief force from the south failed to reach the place in time, and April 29, 1916, General Townshend was forced by starvation to surrender.