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Rh not many is protected by permanent fortification of some sort. Its most vulnerable section is that at which the E. Prussian frontier makes contact with Augustowo-Suwalki-Kovno-Grodno. South of this region, on the stretch Rozan-Lomsha, owing to the absence of railways and first-class roads, military operations were never principal, but always dependent upon either those of Suwalki and Augustowo or those astride the Warsaw-Mlava- Danzig line. North of Kovno, at the broad entrance to the corridor, it was safe against all but secondary attacks, so long as Kovno held out and kept the attack toward Shavli.

Frontally, of course, the corridor was protected by the Vistula and its fortresses Ivangorod, Warsaw and Modlin or Novo- georgievsk (this last at the origin of the flank barrier just de- scribed), and behind this frontal defence were other successive lines the middle Bug, the middle and upper Narew, the upper Niemen and its feeders, the Vilia system not to mention partial barriers such as the Wieprz. But most of these rear barriers, in particular the Bug, tend in their upper course to turn south- ward, thus opening to an invader who stands N. of the San a series of successive gates along the inner edge of the corridor, by which penetration is possible to Bialystok or even to Baranovichi. Hence the special importance attaching, in the operations of 1914-5, to the lower San sector and the fortress of Brest-Litovsk.

The southern corridor, unlike the northern, lies partly on one side of the political frontier and partly on the other. No important natural barrier prevents either an Austrian irruption from the S. as far (roughly) as the line Lublin-Kovel-Sarni, or a Russian irruption through and past Lemberg (Lvov) to the Dniester. As has just been mentioned, the left wing of such an Austrian irruption has the opportunity of seizing the gates of the northern corridor; no reciprocal advantage offers itself to the Russians since the Dniester line is doubled by that of the Carpathians. But, in particular, the fact that the whole Lem- berg region is within the Austrian frontier narrowed the corridor normally open to the Russians to a mere strip of country. To protect this from being cut off from behind, the Russians had constructed a triangle of fortresses Rovno-Dubno-Lutsk. At its front end, where it joins the northern corridor and the salient, Ivangorod, Brest-Litovsk, and minor river courses and marshes were relied upon to seal the region of Chelm and Vladimir Volynsk; in effect, a drive by the Austrians into that region if pressed too deep laid open its flanks to counter-attack both from Ivangorod and from Lutsk (Luck).

The geography of the interior of the marsh area needs little description. As above mentioned, much of it is tactically penetrable, but owing to the extreme paucity of communications, as well as to its physical difficulties, it is on the strategic plane essentially an obstacle and not a field of manoeuvre. Its out- standing geographical feature is its river system; the Pripyat itself runs W.-E., but it has numerous N.-S. tributaries notably on the S. side, and these tributaries sometimes form, with tribu- taries of the Dniester (flowing in the opposite direction), N.-S. waterlines of defence only broken at the watershed (Brody, for example) along which run the communications between Rovno and Lemberg.

In the forepart of the central salient, too, it is the waterlines that are the most important features. The course of the upper Warta; that of the Pilitca; the position of Lodz (or rather Len- czyska) at the divide of the Warta and Bzura systems; the course of the Nida meeting at its mouth the mouth of the Dunajec, one of the several Galician rivers which double the San obstacle; lastly, the upper Vistula itself which forms the southern boun- dary of the salient all these were important.

Practically the whole of this region belongs to the W. Russian plain, and has marshy valleys, feeble undulations, and great forests, some of these last still existing in primeval- density, others already broken up by man's clearings and settlements. The only hilly mass is the Lysa Goza in the Kielce region of the salient. On the contrary, the Lemberg-Brody-Buczacz portion of the southern corridor, and all country between the San or Dniester and the Carpathians, is almost wholly a country of deep-cut valleys and high plateaux.

The German reentrant opposed to the Polish salient is geographically similar to, but in point of human development very different from, that region. In Silesia, owing to its industrial character, the network of roads and railways is as dense as in western Europe. Without going west of Posen, no less than three complete lateral or circumferential railways join Upper Silesia to the trans-Vistula railways of E. Prussia. As, in face of these, no Russian lateral exists W. of Lodz it is easy to see how this region, in spite of its want of natural defences, was able to act as a curtain between the two bastions of E. Prussia and Galicia, facilitating quick transfers of the centre of gravity from flank to flank and itself (save at one critical moment) immune from attack because of the difficulty of approach.

Of these two " bastions," E. Prussia was the more important as menacing the whole length of the northern corridor, from front to rear. Whereas the Lemberg region only projects from the San-Dniester barrier, E. Prussia has its whole length at right angles to the Vistula. It is served by so many railways that either end of this length is utilizable for the offensive.

The principal directions which this offensive may take are from the eastern end of the province towards Shavli, from the same towards Kovno and Grodno, and from Mlava towards the Narew and, if and when that obstacle is overcome, on Siedlce or Bialystok. We have seen that the first of these is inevitably a secondary or dependent operation. Between the other two the choice was always, for the German Command, difficult. Presuming the Narew forced, or Kovno taken, as the preliminary in either case, the one offensive leads close into the rear of the Warsaw-Ivangorod stronghold, while in the other the corridor is seized far back near its entrance; the choice therefore depended on how deeply the enemy was advanced in the Polish salient or how long the passive front of the " curtain " could be held, or what chance there was of cooperation from the lower San through the Bug " gate," and on other factors which had to be reckoned together on every occasion that an offensive was planned. But these two avenues (Kielce or Warsaw-Mlava, and Vilna-Kovno [or Grodno]-Insterburg) equally serve for Russian offensives, and the defensive charac- teristics of E. Prussia were nearly if not quite as important as its qualities as an offensive base.

The main feature of military geography in E. Prussia is the chain of the Masurian lakes which, in a sickle from N. to S. and then westward, protects the interior against attack from the E. or the S.E. The tongues of land which separate the lakes represent only a narrow frontage which has actually to be defended, and have the effect also of gathering communications, plentiful in the interior, at a few points of exit. To the S. of the lakes a number of tributaries of the Bobr-Narew system continue the water barrier, as against eastern attack, to the Narew; to the N. of them the river Angerapp presents a similar barrier as far as the Pregel, beyond which river smaller streams continue the line of defence with some gaps to the Niemen. Behind the lakes, the next important N.-S. barrier is the line of the Alle which, rising in the central Masurian lakes, runs to the Pregel at Wehlau, whence from Tapiau to the Kurische Haff runs the Deime. Other partial barriers to an invader's west- ward progress exist but are of less importance. Finally there is the German section of the lower Vistula which, intricate at Danzig and fortified at Thorn and Graudenz, still bars access to Germany proper when E. Prussia has been conquered or evacuated.

Thus on the E. this province is singularly well protected. But it is to be noted (i) that the frontier, especially in the north- ern part, lies well in advance of the barrier, and that a policy of passive defence on the lake line forfeits a not inconsiderable region at the outset; and (2) that both the Insterburg-Johannis- burg line and the Alle are turned by attack from the S., by Mlava and Soldau, where the "westernmost part of the lake sys- tem dies away. At the centre of the " sickle," on the other hand, the density of the lakes is highest and they not only afford local protection to this part of the region, but also enable the defending army to shift its weight from E. to S.W. and vice