Mouse-holing is a tactic used in urban warfare, in which soldiers create access to adjoining rooms or buildings by blasting or tunneling through a wall. This tactic is used to avoid open streets where advancing infantry, caught in enfilade, are easily targeted by machine-gun and sniper fire.
This tactic was used by British soldiers in Dublin during the 1916 Easter Rising. Mouse-holing began to appear in military tactical manuals in World War II. With mouse-holing, combatants are able to move around an urban battlefield under cover, without needing to expose themselves to enemy fire or observation. A typical passage is large enough for a single file of soldiers. Large, unrestricted holes can compromise the structural integrity of the building, and offer little cover from opposing forces.
During the Battle of Ortona, the Canadian Army, which gave the tactic its name, used it to great effect. Using weapons such as the PIAT or anti-tank guns to breach the walls of a building (as houses within Ortona shared adjoining walls), the soldiers would then throw in grenades and assault through the mouse holes, clearing the top floors and making their way down, where both adversaries struggled in repeated close-quarters combat. Mouse-holing was also used to pierce through walls into adjoining rooms, sometimes catching enemy troops by surprise.
Similar to underground tunnels used in rural battlefields, mouse-holes can also allow forces to infiltrate behind enemy lines, providing a significant tactical advantage. In some cases, a mouse-hole will be camouflaged with furniture, especially when they are created to aid a defending force or a clandestine operation. When used in defensive positions, mouse holes often join and combine with underground tunnels. This was used by the Red Army of the Soviet Union during the Battle of Stalingrad, where it allowed troops to consistently infiltrate areas to the German rear that were supposedly cleared. The ubiquitous availability of the Panzerfaust in the last months of the war made all sides use it to quickly breach buildings from unexpected directions.
The tactic was used heavily by anti-coalition insurgents during the Iraq War who would connect houses converted into fortified bunkers by creating holes in walls in order to evade and ambush coalition troops. In addition, coalition snipers would utilize mouse-holing as a method of being able to fire at enemy fighters from further within rooms and other structures, thereby concealing their position. 
Mouse holes can be made in light interior walls by hand or with small arms. More substantial walls require the use of explosives such as a satchel charge, or a large caliber, vehicle-mounted cannon or tank gun. If time and conditions allow, breaches can be made with even small amounts of explosive if properly tamped and braced, e.g. with sandbags and props, to direct the force of the explosion into the wall. Since the early 1990s many armies developed special purpose weapons like the MATADOR and SMAW specifically for this tactic.
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