In 2005, Frieser wrote that Allied planning was notable for its failure to cut the panzer corridor when there were insufficient German motorised divisions to consolidate behind the panzer divisions and the non-motorised divisions were several days' march away. The corridor was only wide at Arras and vulnerable to a pincer movement; The Allies could have achieved a Clausewitzian counter-encirclement and cut off the panzers on the channel coast. Halder had been willing to run the risk, because of the incapacity of the Allies to achieve a fast tempo of operations. The Allied attack was contained because Rommel improvised a gun line of anti-tank guns and anti-aircraft guns, that managed to stop some of the lighter British tanks; the infantry tanks, particularly the Matilda IIs, proved very difficult to knock out with the standard 37 mm anti tank gun.