 Operation Tractable began at 12 : 00 on 14 August , when 800 Avro Lancaster and Handley Page Halifax heavy bombers of RAF Bomber Command struck German positions along the front . As with Totalize , many of the bombers mistakenly dropped their bombs short of their targets , causing 400 Polish and Canadian casualties . Covered by a smoke screen laid down by their artillery , two Canadian divisions moved forwards . Although their line of sight was reduced , German units still managed to inflict severe casualties on the Canadian 4th Armoured Division , which included its Armoured Brigade commander Brigadier Leslie Booth , as the division moved south toward Falaise . Throughout the day , continual attacks by the Canadian 4th and Polish 1st Armoured Divisions managed to force a crossing of the Laison River . Limited access to the crossing points over the Dives River allowed counterattacks by the German 102nd SS Heavy Panzer Battalion . The town of Potigny fell to Polish forces in the late afternoon . By the end of the first day , elements of the Canadian 3rd and 4th Divisions had reached Point 159 , directly north of Falaise , although they had been unable to break into the town . To bolster his offensive , Simonds ordered the Canadian 2nd Infantry Division to move toward the front , with the hope that this reinforcement would be sufficient to enable his divisions to capture the town . 
