The ÁrpáÁrpád Line (; ; ) was a line of fortifications built in 1941-44 in the north-eastern and eastern Carpathian Mountains, along the border of Kingdom of Hungary. It was named after Árpád, the head of the Hungarian tribes. The main function of the Line was to protect Northern Transylvania (including Székely Land) and Carpathian Ruthenia from the east.
The Árpád Line was the main line of defence; the Hunyad and Prinz Eugene Lines lay in front of it.
Theoretical background: the flexible defense
General Teofil Hárosy (pioneer) developed the theory of flexible defense in 1939-1940. "Flexible" defense-lines (Mannerheim Line, Árpád Line, Bar Lev Line) were not based on dense lines of heavily armed, large and expensive concrete pillboxes (as the Maginot and Siegfried systems were). Their protective capacity hinged on multiple lines of well-designed obstacles fitting into the environment. All roads passable by tanks and trucks were closed by a "völgyzár", but the intermediate areas were unprotected. The flexible defense only closed possible supply lines, thus the enemy could bypass the fortified line, but every battlegroup without resupply quickly ran out of ammo and food. Thus, the Árpád line was based on small, circularly defensible fortified zones, rather than a continuous impenetrable line of fortresses. Most of the roads were destroyed in the forested, mountainous and swampy areas between forts. Therefore, the task of defenders was to protect the obstacles, and not fight with the main Soviet forces.
The main method of this type of field fortification was to close potential vehicular transport and attack routes with multiple anti-tank ditches, hedgehogs, dragon's teeth and mine fields. These obstacle-zones are followed by a quite small but peculiarly complex system of ditches and barbed wire obstacles, which protects the anti-tank barrier against sappers, bridge-layer tanks and engineer teams. The semi-mobile engagement is the main point, soldiers fight in open firing positions, bunkers served as only shelters during bombardment. Thus, it is possible to quickly reallocate soldiers between the points of defense (manpower not locked in pillboxes), and counterattacks feasible by defense force. This type of defensive line required 5-10 times less manpower and concrete per kilometer than the Maginot or Siegfried Lines, with equal (or more) effectiveness; but far more imagination was required from the builders because every part of the line had an inevitably unique design, adapted to local conditions.
The main defense line usually lies on a hilltop on the edge of a forest, and divided into circularly defensible independent sectors (platoon size). A sector includes 3 (or more) trench-rings and 1 or 2 ferro-concrete shelter(s) surrounded by 30 m (or more) wide zone of anti-personnel obstacles (barbed wire, electric fences, wide variety of non-explosive traps) and 2 (or more) "V" or "U" shaped anti-tank trenches or anti-tank walls. Independent sectors were connected by communication trenches into a defensive zone or fortress (Hungarian: völgyzár – literally "valley fort").
Unlike the Maginot and Siegfried-type lines, the overall barrier and trench system of flexible defense lines were hard to detect. It was a complex net of small objects, and all of them were entirely adapted to the natural topography (e.g., natural steep slopes as anti-tank wall, trenches curving as isolines). All buildings, dugouts and obstacles are adapted to the landscape, and there are no easily targetable large objects, while Maginot-like bunkers were impossible to be camouflaged enough.
Therefore, the enemy is forced to attack the defensive trenches with conventional infantry assault (as during World War I) at the cost of terrible losses, without assistance of armored forces or line-of-sight firing guns. It is a "flexible" type of defence line because soldiers are not locked in bunkers, the defensive platoons could be regrouped between open firing positions (firing positions and dugouts made of wood and earth) and independent sectors. Furthermore, soldiers can easily carry out a counterattack (supported by heavy weapons of the Line) optionally. Because of the possibility of redeployment, significantly less manpower is required than Maginot-like defenses. The semi-mobile engagement was also psychologically preferable to the claustrophobic feeling rendered by Maginot-like bunkers in the long term.