German units within the fortress withstood probing attacks and as the opening of the port of Antwerp was more important, the 21st Army Group commander, Field Marshal Bernard Montgomery, decided to contain but not capture Dunkirk with the 1st Czechoslovak Armoured Brigade.

Under orders from Dwight Eisenhower, the Allied Supreme Commander, Montgomery modified his instructions to the Canadian commander, Henry Crerar, on 13 to 14 September, "Early use of Antwerp so urgent that I am prepared to give up operations against Calais and Dunkirk" and "Dunkirk will be left to be dealt with later; for the present it will be merely masked" (the garrison prevented from mounting sorties).

[1] Action against Calais continued in Operation Undergo, at least partly due to the need to silence the heavy artillery nearby.

The German outer perimeter ran through the villages of Mardyck, Loon-Plage, Spycker, Bergues and Bray-Dunes, 4.3 to 7.5 mi (7 to 12 km) from Dunkirk.

The Calgary Highlanders attacked Loon-Plage on 7 September against very heavy opposition and suffered enough casualties that each of its companies was reduced to less than 30 men.

Over the next ten days, Canadian units nibbled away at the German perimeter, taking Coppenaxfort on 9 September, Mardyck on 17 September, both west of the city, Bergues on the 15th and Veurne, Nieuwpoort (assisted by the Witte Brigade (White Brigade) part of the Belgian Resistance) and De Panne, east of Dunkirk, in Belgium.

Given the need to open the Scheldt estuary to Antwerp and the likelihood that Dunkirk would be of limited use as a port as a result of its demolition, the Canadian units were redeployed.

Nearby Ostend (Oostende) had fallen easily to the Canadians when the Germans withdrew and its port was partially opened on 28 September, easing the Allies' supply problems.

[5] The Allied forces around Dunkirk were to contain the German garrison and minimise their inclination to fight on by reconnaissance, artillery and air bombardment and with propaganda.

[8] The Germans attempted to take advantage of the change with sorties against the 7th Black Watch in Ghyvelde and against 7th Argylls nearby at Bray-Dunes Plage.

The low-lying ground outside the city had been flooded to form part of the defences and adjacent land easily became waterlogged, hampering movement and making life unpleasant.

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