The WW II Allied campaign in Italy is often called “an Engineer’s War” because of both the terrain and the enemy-created obstacles that had to be overcome. In December 1943 the German Army had retreated to the "Bernhard Line" across the narrowest part of the Italian Peninsula in an attempt to halt the Allied advance. This area was crossed by many mountain rivers that were swollen with the winter rains and the German military engineers had reinforced these natural obstacles by demolishing most of the bridges in the area south of this defensive line. They had also cratered road defiles, torn up railway tracks, and planted many minefields. Furthermore, the obstacle effect of the defended towns and villages was reinforced by even more demolitions and the use of mine and booby-traps.
At this stage, the Canadian engineer resources of the 1st Canadian Division were already fully committed to restoring the routes by building bridges, clearing minefields (that were often booby trapped) and other tasks to allow the forward movement of the tanks and heavy transport vehicles. The Infantry Assault Pioneers were helping with breaching the minor obstacles.
On 21 December 1943 the Loyal Edmonton Regiment and the Seaforth Highlanders entered Ortona, the eastern anchor of the Bernhard Line, supported by the tanks of the Three Rivers Regiment. Here they encountered what has been called the most intensive street fighting in the history of warfare to that time. The Engineers and Infantry Assault Pioneers were already heavily involved in clearing obstacles that were restricting the advance but much of the battle now involved intense house-to-house fighting with a determined enemy.
Fighting from building-to-building was both a time-consuming and dangerous task. Not only were the Allied soldiers exposed to the dangerous close-in fights within the buildings, they were exposed to enemy fire from down the street when they exited a cleared building to move to the next one.
It was in these conditions that the tactic of "mouse-holing" was perfected by combining the ideas from the Engineers and Infantry Assault Pioneers. In concept, a hole was prepared in the common wall between adjacent houses to gain entry to the second house from within. This breach was accomplished either by hand tools or by explosives. If the enemy occupied the upper floors a demolition charge was generally used that quickly destroyed the house. This procedure allowed the Canadians to advance up the street unseen by the enemy. This technique and tactic proved successful and was adopted by other Allied armies.
On 27 December the German Army broke off contact and the Battle of Ortona was won – with some particular credit to the technique of ‘mouse-holing.’