In 1949, Churchill wrote that the defence of Calais led by Nicholson delayed the German attack on Dunkirk, helping to save the British Expeditionary Force, a claim that German general Heinz Guderian contradicted in 1951. In 1966, Lionel Ellis, the British official historian, wrote that three panzer divisions had been diverted by the defence of Boulogne and Calais, giving the Allies time to rush troops to close a gap west of Dunkirk. In 2006, Karl-Heinz Frieser wrote that the halt order issued to the German unit commanders because of the Anglo-French attack at the Battle of Arras (21 May) had a greater effect than the siege. Hitler and the higher German commanders panicked because of their fears of flank attacks, when the real danger was of the Allies retreating to the coast before they could be cut off. Reinforcements sent from Britain to Boulogne and Calais arrived in time to forestall the Germans and hold them off when they advanced again on 22 May.