Through this region the armies must cross big rivers: the Oder, Dvina, Warthe, Vistula, Pregel, and Niemen, northward and northeastward. Just above or eastward of that point, where the German-Russian frontier touches the shore, the Baltic curls into a dent, 100 miles deep, forming the Gulf of Riga. Near the southern extremity of this gulf, eight miles from the mouth of the Dvina, is the city of Riga, ranking second only to Petrograd in commercial importance as a seaport, and with a population of about 300,000. As the armies move across the frontier they come to a vast domain projecting into this marsh country, like a great, broad tongue licking the shore of the Baltic; this wide strip of German territory is East Prussia--a country to be beleaguered. Not far below the tip of this tongue, about five miles from the mouth of the Pregel River in the Frische Haff, and about twenty-five miles from the seacoast, is situated another embattled stronghold--the city of Koenigsberg which, since 1843, has been a fortress of the first rank. These two cities in the following pages will be the immediate objectives of the enemy forces operating on this section of the eastern front. It will be obvious why the lines of battle were less permanently fixed here than in the more solid and mountainous sections of northern France. Railroads and fairly well-laid highways do indeed traverse these swamps in various parts, especially in German territory, but trenches could not be dug in yielding mire. In yet another feature were the military operations hampered by the nature of the terrain here; the use of heavy artillery. We have seen that one of the chief causes of success attending German attacks in the other theatres of the war has been their use of heavy guns. But in the fighting before Riga, we shall see when the Germans seemed on the point of taking that city their heavy artillery was so handicapped that it was rendered practically useless. Being restricted by the marshes to an attack over a comparatively narrow front, they were compelled to leave their heavy guns behind on firmer soil. The guns which they could take with them were matched by the Russians; the fighting was, therefore, almost entirely limited to infantry engagements, in which the Russians were not inferior to the Germans. Thus, we shall find the German advance on Riga was stopped before it could attain its object.