path of the German spearhead. On 16 May, Georges realised that the Panzer divisions might reach the coast and outflank all the Allied armies to the north of them. He asked for the 23rd Division to defend the Canal du Nord at Arleux. The British Staff was of the opinion that the German breakthrough consisted of small detachments of light reconnaissance troops and that using these lightly armed and largely untrained troops against them did not seem unreasonable. The area was otherwise devoid of Allied units, so there was little alternative. The three divisions were grouped together in an improvised corps called Petreforce and on 18 and 19 May, the Territorials, lacking motor transport, began to march or entrain towards their defence positions. The 70th Brigade of the 23rd Division dug in on the Canal Line but was ordered to withdraw towards Saulty on 20 May; in the process they were caught in the open by elements of 6th and 8th Panzer Divisions, from which only a few hundred survivors escaped. The 69th Brigade defended Arras and the 12th Division fought to delay 2nd Panzer Division on the Canal Line near Arras, at Doullens, Albert and Abbeville. The 138th Brigade of the 46th Division fought on the Canal Line but the 137th Brigade trains were attacked by the en route; the survivors were able to withdraw to Dieppe and later fought on the Seine Crossings. The 139th Brigade fought on the River Scarpe and later defended the Dunkirk perimeter. By the end of 20 May, the divisions had ceased to exist, in most cases having only delayed the German advance by a few hours. 21–23 May The push by Army Group A towards the coast, combined with the approach of Army Group B from the north-east, left the BEF enveloped on three sides and by 21 May, the BEF had been cut off from its supply depots south of the Somme. The British counter-attacked at the Battle of Arras on the same day. This was well to the south of the main BEF force on the Escaut, where seven BEF divisions were placed in the front line. The British divisions were facing nine German infantry divisions, who began their attack on the morning of 21 May with a devastating artillery barrage. Shortly afterwards, infantry assaults started along the whole front, crossing the canalised river either by inflatable boats or by