From a strategic point of view, the northern frontier was the most difficult of all. It passed north of Kilimanjaro, to the west of which is a desert belt. East of this desert belt and Kilimanjaro the enemy colony was protected by an almost impassable mountain system, with a very narrow, swampy, dangerous gap between the Usambara and Pare Mountains, and another gap of about four or five miles between the Pare Mountains and Kilimanjaro. It was impossible to move an army through the first gap; the second gap at the foot of Kilimanjaro was the place where the enemy had located himself early in the war on British territory, and with patience and skill had dug himself in, with very extensive fortifications, surrounded by dense forests and impassable swamps. Here he lay waiting for eighteen months, threatening British East Africa. From here he was driven in March, 1916, and by the end of that month our forces had conquered the whole Kilimanjaro-Meru areas. It was at this stage, and after our initial success, that the rainy season set in; and that is another great feature of German East Africa. I had read much about it, and I had heard more; but the reality far surpassed the worst I had read or heard. For weeks the rain came down ceaselessly, pitilessly, sometimes three inches in twenty-four hours, until all the hollows became rivers, all the low-lying valleys became lakes, the bridges disappeared, and all roads dissolved in mud. All communications came to an end, and even Moses himself in the desert had not such a commissariat situation as faced me. [Sidenote: The enemy's line of retreat.] When in the latter part of May the rains subsided, the advance against the enemy was once more resumed. In order to create the maximum difficulties for our advance, the enemy chose as his line of retreat the great block of mountains which I have referred to as forming the eastern buttress of the great central plateau. For the next three and a half months our forward movement continued with only one short pause until by the middle of September we had reached the great valleys of the Rufiji and the Great Rwaha in the far south, and across the Rwaha we could link up with General Northey at Iringa in the southwest. [Sidenote: Difficulties of transport and supply in advance.] [Sidenote: Poisonous insects and tropical diseases.] [Sidenote: The campaign a story of human endurance.]