The combined British, French and Belgian forces were rapidly split around Armentičres. The German forces then swept north to capture Calais, holding a large body of Allied soldiers trapped against the coast on the Franco-Belgian border. It became clear to the British that the battle was lost and the question was now how many Allied soldiers could be removed to the relative safety of England before their resistance was crushed.
From May 22 preparations for the evacuation began, codenamed Operation Dynamo, commanded from Dover by Vice-Admiral Bertram Ramsay. He called for as many naval vessels as possible as well as every ship capable of carrying 1,000 men within reach. It initially was intended to recover around 45,000 men of the British Expeditionary Force over two days, this was soon stretched to 120,000 men over five days. On May 27 a request was placed to civilians to provide all shallow draught vessels of 30 to 100 feet for the operation, that night was the first rescue attempt. A large number of craft including fishing boats and recreational vessels, together with Merchant Marine and Royal Navy vessels, were gathered at Sheerness and sent to Dunkirk and the surrounding beaches to recover Allied troops. Due to heavy German fire only 8,000 soldiers were recovered.
Another ten destroyers were recalled for May 28 and attempted rescue operations in the early morning but were unable to closely approach the beaches although several thousand were rescued. It was decided that smaller vessels would be more useful and boatyards were scoured for suitable craft, gathering them at Sheerness, Chatham and Dover. The Allied held area was reduced to a 30 sq km block by May 28. Operations over the rest of May 28 were more successful, with a further 16,000 men recovered but German air operations increased and many vessels were sunk or badly damaged, including nine destroyers.
On May 29 there was an unexpected reprieve, the German armour stopped its advance on Dunkirk leaving the operation to the slower infantry, and the Luftwaffe (Hermann Göring, then in great favour with Adolf Hitler, had promised air power alone could win the battle) but due to problems only 14,000 men were evacuated that day. On the evening of May 30 another major group of smaller vessels was dispatched and returned with around 30,000 men. By May 31 the Allied forces were compressed into a 5 km deep strip from La Panne, through Bray-Dunes to Dunkirk, but on that day over 68,000 troops were evacuated with another 10,000 or so overnight. On June 1 another 65,000 were rescued and the operations continued until June 4, evacuating a total of 338,226 troops aboard around 700 different vessels.
Until the operation was complete the British prognosis had been gloomy, with Churchill warning the House of Commons to expect "hard and heavy tidings". Subsequently the British Prime Minister referred to the outcome as a "miracle" and exhortations to the "Dunkirk spirit" - of triumphing in the face of adversity - are still (occasionally) heard in Britain today.