Kessel (cauldron)

A salient is a battlefield feature that projects into enemy territory. The salient is surrounded by the enemy on three sides, making the troops occupying the salient vulnerable. The enemy's line facing a salient is referred to as a re-entrant (an angle pointing inwards). A deep salient is vulnerable to being "pinched out" across the base, forming a pocket in which the defenders of the salient become isolated. On the other hand, a breakout from the salient's tip can threaten the defender's rear areas and leaves neighbouring sections of their line open to attack from behind.

Salients can be formed in a number of ways. An attacker can produce a salient in the defender's line by either intentionally making a pincer movement around the flanks of a strongpoint, which becomes the tip of the salient, or by making a broad, frontal attack which is held up in the centre but advances on the flanks. An attacker would usually produce a salient in his own line by making a broad, frontal attack that is successful only in the centre, which becomes the tip of the salient.

In trench warfare, salients are distinctly defined by the opposing lines of trenches, and they were commonly formed by the failure of a broad frontal attack. The static nature of the trenches meant that forming a pocket was difficult, but the vulnerable nature of salients meant that they were often the focus of attrition battles.

In mobile warfare, such as the German Blitzkrieg, salients were more likely to be made into pockets which became the focus of annihilation battles.

A pocket carries connotations that the encircled forces have not allowed themselves to be encircled intentionally, as they may, when defending a fortified position which is usually called a siege. This is a similar distinction to that made between a skirmish and pitched battle.

Soviet military doctrine distinguishes several sizes of encirclement:

The significances of these terms are reflected by the conception of what can be expected in combat in encirclement operations. A cauldron is expected to be “boiling” with combat activity, the large enemy forces still quite able to offer “hot” resistance in the initial stages of encirclement, and so are to be contained, but not engaged directly. A sack in Soviet experience was often created as a result of operational breakthroughs, and was sometimes as unexpected for the Soviet command as for the enemy. This encirclement, sometimes of an entity of unknown size, tended to move for some time after the initial encirclement due to inherently dynamic nature of operational warfare. By contrast a nest was a reference to a tactical, well-defined and contained encirclement of enemy troops that was seen as a fragile construct of enemy troops unsupported by its parent formation (the use of the word nest, is similar to the more familiar English expression machine gun nest).

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Wikipedia