It would appear, therefore, that Russian Poland would offer excellent conditions for an army on the defensive. And this is quite true, the Vistula, especially, serving as a screen against the attacking armies from the west. As a matter of fact, it would have been extremely difficult to take Warsaw by a frontal attack. Warsaw's weakness lay in the north in the swamp regions. One of the greatest dangers in all wars, against which a military commander has to guard his army, is that of being flanked. The road or roads leading from the rear to the base of supplies, along which not only food supplies for the soldiers, but, quite as important, ammunition, is brought up, either in wagons, automobiles, or in railroad trains, are the most sensitive part of an army's situation. Unless they are very short--that is, unless an army is very close to its base of supplies--it is impossible to guard these lines of communication adequately. Therefore, if the enemy is able to break through on either side of the front, there is great danger that he may swing his forces around and cut these lines of communication. The army that is thus deprived of its sources of supply has nothing left then but to surrender, sometimes even to inferior forces. Sometimes, of course, if the army is within the walls of a fortified city and is well supplied with food and ammunition, it may hold out and allow itself to be besieged. This may even be worth while, for the sake of diminishing the enemy's strength to the extent of the forces required for besieging, usually many times larger than the besieged force. But in the case of Warsaw we shall see that that would not have been a wise plan; hardly any food supply that could have been laid by would have maintained the large civil population, and the big guns of the Germans would soon have battered down the city's defenses. This the Russians realized from the very beginning. As is well known now, Russia had never intended to hold Poland against the Teutons. Her real line of defense was laid much farther back. It was only on account of the protest of France, when the two Governments entered into their alliance, that any fortifications at all were thrown up in Poland. A real line of defense must be more or less a straight line, with no break. And the marshes in the north, as well as the tongue of East Prussia projecting in along the shores of the Baltic toward Riga made that impossible. Russia's real line of defense was farther east, along the borders of Russia proper and along the line of railroad already referred to. By studying this territory east of Poland it will become obvious why Russia should prefer this as her main line of defense against a German invasion.