A great European war of the future will, it may be assumed, be fought on one or the other frontier of Germany--in the Franco-German area on the western side; or the German-Austro-Russian area on the eastern--or on both. Since it would be impossible under modern conditions for Germany, with or without Austrian co-operation, to invade both France and Russia, she would be obliged to defend one frontier while crossing the other. An attack upon France would involve the traversing of a difficult stretch of country in which elaborate arrangements have been made for defence; and although the French army is not so strong as that of Germany, it would have the enormous advantage of standing on the defensive. Even if Germany were to gain initial successes through her superior swiftness in mobilization, the difficulties of modern warfare are such that she could not hope, even under abnormally favourable circumstances, to capture Paris in less than two years, and long before then she would be reduced to a state of entire economic exhaustion. It is to be borne in mind that the invading army would constantly grow weaker, while the defenders would be able to enforce the superiority now belonging to defence by bringing up all their reserves. Difficulties which would be, if possible, even harder to surmount would attend a French attempt to invade Germany. The elaborate plans that have been drawn up for an Austro-German invasion of Russia would, in all probability, be doomed to failure. The defensive system of Russian Poland is regarded as almost perfect. Even if the German and Austrian forces could evade the Polish defences, they would waste their strength against the second Russian fortified line; and even if that were broken through, St. Petersburg and Moscow would still be far distant, and Russia's immense resources in men would enable her to bring up body after body of reserves against the dwindling invading force. A Russian invasion of Prussia would have to encounter an elaborately scientific defensive system, and would be liable to all the other difficulties to which an invasion is exposed--particularly, in this case, the difficulty of feeding a vast host of men on hostile territory. The weakness of Austria's Galician frontier seems tempting; but Russia would have to strike at Germany--an invasion of Austria which left Germany untouched would be mere waste of energy. The general conclusion is that invasion of an enemy's country, in a great European struggle, would, in all probability, lead to the destruction of the invaders and the entire exhaustion of both combatants. _III.--Modern War at Sea_