the enemy step by step in minor skirmishes and ambushes carried out by forces supplied and supported by central fortresses not located in the front line. In short, one theory advocated many minor fortifications built to directly engage the enemy, while the other advocated only a few major fortifications built not to directly engage the enemy, but to support the troops that engaged the enemy. In the beginning of the 19th century the peripheral system—at least in Sweden—was deemed unmodern, due to the extremely long border and coastline of the country, which required a lot of personnel to maintain and support, leading to high costs and a neglect of the more mobile armed forces. The peripheral system was also meant to contain the enemy to a specific area, the coast and borders, but as time went and new tactics were taken into practice, the thin line of fortifications would be easily penetrated by the enemy forces, and when the enemy had advanced through the peripheral line, there would not be enough troops in the hinterland to stop the attacker. The central fortress system was better adjusted to defence in depth, it was not until the enemy attack had reached its culminating point that full counter-attacks would be launched. The central defence had also proven effective in the Seven Years' War—used by Frederick II of Prussia—and in Napoleon's Invasion of Russia—used by the Russians. Carl von Clausewitz, in his military treatise On War, even mentioned Sweden's good conditions for this kind of war of attrition: The fourth principle, the Assistance of the Theatre of War, is naturally an advantage on the side of the defensive. [...] the army on the defensive continues to keep up its connection with everything, that is, it enjoys the support of its fortresses, is not weakened in any way, and is near to its sources of supply. [...] The campaign of 1812, gives as it were in a magnifying glass a very clear illustration of the effect of the means specified under principles 3 and 4. [...] It is true that with the exception of Sweden there is no country in Europe which is situated like Russia, but the efficient principle is always the same, the only distinction being in the greater or less degree of its strength. Fortification of Sweden Sweden had just lost one third of its population, as Russia had conquered the eastern