The offensive of Lieutenant General Sir Oliver Leese’s British 8^th Army on the 'Winter Line' defences to the east of the Apennine mountains had begun on 23 November with the crossing of the Sangro river. By the end of the month, the main 'Gustav-Linie' defences, which was the major part of the three-line 'Winter Line', had been penetrated and Allied troops were fighting their way forward to the next water line, the Moro river 4 miles (6.4 km) to the north of whose mouth lies Ortona. For the Moro river crossing early in December Major General C. F. Keightley’s exhausted British 78^th Division on the Allied right flank along the Adriatic coast had been relieved by the Canadian 1^st Division. By the middle of December, after fierce fighting in the cold and mud, the Canadian 1^st Division’s 1^st Brigade, under the command of Brigadier H. D. Graham, had fought its way to within 2 miles (3.2 km) of Ortona and was then relieved by Brigadier B. M. Hoffmeister’s 2^nd Brigade for the advance on the town.

Some historians suggest that Ortona was of considerable strategic significance as it was one of only a few usable deep-water ports on the Italy’s eastern coast, and was needed for the docking and unloading of Allied ships and thus the shortening of the 8^th Army’s lines of supply, which at the time stretched back to Bari on the 'heel' and Taranto in the 'arch' of the Italian 'foot'. Allied forces were ordered to maintain the offensive, and forcing a way through the built-up areas in and around Ortona was the only feasible option. Ortona was part of the 'Winter Line' defence system, however, and the Germans had constructed a series of interlocking defensive positions in the town. Together with the fact that the Germans had been ordered to 'fight for every last house and tree'. this rendered the town a formidable obstacle to any attacking force.

Other historians have assigned a lesser importance to Ortona. Generalfeldmarschall Albert Kesselring, the German commander-in-chief in Italy, said 'We do not want to defend Ortona decisively…but the English have made it appear as important as Rome.' General Joachim Lemelsen, the temporary commander of the 10^th Army, replied that 'It costs so much in blood, it cannot be justified.' Nonetheless, the Allies believed it would be merely a minor battle and proceeded with the plan; the Germans then rose to the occasion, holding the town with great determination. The Canadians faced elements of the 1^st Fallschirmjägerdivision, whose men were battle-hardened after many years of war, and defended doggedly.

The initial Canadian attack on Ortona was delivered on 20 December by the Canadian 2^nd Brigade’s Loyal Edmonton Regiment with elements of The Seaforth Highlanders of Canada under command. Meanwhile, elements of the same division’s 3^rd Brigade, commanded by Brigadier T. G. Gibson, launched a northerly attack to the west of the town in attempt to outflank and cut off the Germans' rear communications, but made onlt slow progress because of the difficulties of the terrain and the skillful and determined German defence. On 21 December 1943, the Loyal Edmonton Regiment and the Seaforth Highlanders entered Ortona, assisted by the tanks of the Three Rivers Regiment, part of the Canadian 1^st Armoured Brigade under the command of Brigadier R. A. Wyman.

The Germans had concealed various machine guns and anti-tank emplacements throughout the town, making movement by armour and infantry increasingly difficult. The house-to-house fighting was vicious and the Canadians made use of a tactic that had previously been used only infrequently: this 'mouse-holing' tactic involved the the use of weapons such as the PIAT anti-tank projector or cumbersome Teller anti-tank mine to blow a large hole in the wall of a building, and this worked well in Ortona, where the houses shared adjoining walls. The Canadians would then throw in grenades and make their assault through the mouse holes, clearing the stairs to the top or bottom floor with grenades and/or machine guns. They would then pursue to locate and destroy any Germans in what was a repeated struggle of close-quarter combat. Mouse-holing was also used to pierce walls into adjoining rooms, sometimes catching German troops by surprise. The tactic was employed repeatedly as the alternative of assaulting through the streets caused heavy casualties for the Canadian as well as the German troops.

The use of the mouse-holing tactic thus made it possible for the Canadian infantry to progress through the town, building by building, without entering the streets where they would face German fire. While some sources attribute the strategy to the Canadian forces, a British training film of 1941 had already illustrated the concept. The Canadians were certainly early, effective and courageous users of the technique, however. Throughout the 'Battle of Ortona', engineers on each side also employed the brutal but effective tactic of using demolition charges to collapse entire buildings on top of their opponents.

On 28 December, after eight days of fighting, the depleted German troops finally withdrew from the town. The Canadians had suffered the loss of 1,375 men killed during the Moro river battles, of which the 'Battle of Ortona' was one part. This represented almost a quarter of all Canadians killed during the whole Italian campaign. Other sources have placed Canadian casualties as high as 2,300 (including 500 dead) before the town was won for the Allies.

The Canadians destroyed the dome on the cathedral of Santo Tommaso in the town centre using tank fire, in order to prevent its use use for spotting. On 25 December the Allies, who had by then occupied a smaller church, were ordered to destroy both the cathedral and the civilian hospital, but this was largely avoided.