Nevertheless, all military organizations, land or sea, are ultimately dependent upon open communications with the basis of the national power; and the line of communications is doubly of value, because it usually represents also the line of retreat. Retreat is the extreme expression of dependence upon the home base. In the matter of communications, free supplies and open retreat are two essentials to the _safety_ of an army or of a fleet. Napoleon at Marengo in 1800, and again at Ulm in 1805, succeeded in placing himself upon the Austrian line of communication and of retreat, in force sufficient to prevent supplies coming forward from the base, or the army moving backward to the base. At Marengo there was a battle, at Ulm none; but at each the results depended upon the same condition,—the line of communication controlled by the enemy. In the War of Secession the forts of the Mississippi were conquered as soon as Farragut’s fleet, by passing above, held their line of communications. Mantua in 1796 was similarly conquered as soon as Napoleon had placed himself upon the line of retreat of its garrison. It held out for six months, very properly; but the rest of the campaign was simply an effort of the outside Austrians to drive the French off the line, and thus to reinforce the garrison or to enable it to retreat. _Importance of Sea Communications_[26] Except Russia and Japan, the nations actively concerned in this great problem [the problem of Asia] rest, for home bases, upon remote countries. We find therefore two classes of powers: those whose communication is by land, and those who depend upon the sea. The sea lines are the most numerous and easy, and they will probably be determinative of the courses of trade. Among them there are two the advantages of which excel all others—for Europe by Suez, from America by way of the Pacific Ocean. The latter will doubtless receive further modification by an isthmian canal, extending the use of the route to the Atlantic seaboard of America, North and South.