A pocket is a group of combat forces that have been isolated by opposing forces from their logistical base and other friendly forces. In mobile warfare, such as blitzkrieg, salients were more likely to be cut off into pockets, which became the focus of battles of annihilation. The term pocket carries connotations that the encirclement was not intentionally allowed by the encircled forces, as it may have been when defending a fortified position, which is usually called a siege. That is a similar distinction to that made between a skirmish and pitched battle. Implementation Soviet military doctrine Soviet military doctrine distinguishes several sizes of encirclement: Cauldron or kettle ( or kotyel; ): a very large, strategic-level concentration of trapped enemy forces Sack (; ): an operational-level trapped enemy force Nest (; ): a tactical-level trapped enemy force The significance 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). Kessel In German the word Kessel (literally a cauldron though cognate with kettle) is commonly used to refer to an encircled military force, and a Kesselschlacht (cauldron battle) refers to a pincer movement. The common tactic which would leave a Kessel is referred to as Keil and Kessel (Keil means wedge). Kessel is a loanword in English texts about World War II. Another use of Kessel is to refer to Kessel fever, the panic and hopelessness felt by any troops who were surrounded with little or no chance of escape. 'Kettling' is also used to refer to a police tactic in which police