From May 16th 1940, in response to the deteriorating situation in France, Allied front-line troops in Belgium began pulling back from the River Dyle, to a new line along the River Escaut. The following day, the BEF commander, General Gort, lost communications with the Belgians on his left flank and the French 1st Army on his right. By the 23rd, virtually all allied troops had been forced back to the French frontier, having to run the gauntlet of a rampaging Luftwaffe. Gort was under intense pressure from the new French Commander-in-Chief, General Weygand (who had replaced Gamelin on the 20th May) and the British government, to attack south, to link up with French forces on the Somme. Such a strategy ignored the reality that, with the bulk of Gort's forces already heavily engaged with the Germans, to disengage and attempt such an attack, was little short of suicidal.

Behind the scenes, Gort's staff were ordered to draft a plan for the withdrawal of the whole of the BEF from a 27-mile stretch of coastline between Dunkirk and Ostend. By this stage the ports of Boulogne, Calais and Ostend had fallen or were in the process of being lost, which left Dunkirk as the only viable option. Dunkirk was the third-largest port in France and was a fortress town in its own right, under the control of a French commander, Admiral Jean Abrial.

Unbeknown to Gort, the British Navy had already commenced working on their part of the evacuation plan as early as the 19th of May. Responsibility for the operation was passed to Vice-Admiral Ramsey, based in the Admiralty Tunnels under Dover Castle. The operation was code-named 'Operation Dynamo' and was named after one of the underground chambers in the tunnels complex. However, the French, for the time being at least, still clung to the fantasy of using the Dunkirk pocket to maintain a permanent bridgehead in Flanders.

Meanwhile, the bulk of the BEF, the French 1st Army and the remnants of the Belgian army were compressed into a long (60 miles) and narrow (15-25 miles) corridor, running inland from the sea, to Lille in the south. On the 26th May, Gort gave orders to evacuate the BEF. At 6.57pm that same day, Vice-Admiral Ramsey received his orders to commence Operation Dynamo, with the 129 ferries, coasters, Schuyts (scoots) and small craft at his disposal. More vessels, including the valuable destroyer flotilla, were added as the urgency of the task grew.

One of the problems the Navy had was planning the evacuation routes between Dover and Dunkirk. Three routes were eventually used. Route Z, the shortest at 39 miles, skirted south of the Goodwin Sands, before crossing the Channel on a Dover - Calais axis. It then followed the French coast in an easterly direction to Dunkirk. However, by the 27th May, German artillery had established positions on the coast between Calais and Gravelines and effectively closed this route for daylight operations. Route X ran further to the north-east, was longer at 55 miles and was full of dangerous shoals. To compound matters, this route also ran through a major allied minefield through which a channel would need to be swept prior to its use. This left Route Y, which ran even further to the north-east, virtually to Ostend, prior to doubling back westwards at the Kwinte Buoy and the long run into Dunkirk. This route was to be the mainstay for the majority of the evacuation, being easy to navigate, relatively free of mines and out of range of German guns. However, at 87 miles long, it was in excess of double the length of Route Z which meant that twice the number of ships would be required to maintain the planned evacuation schedule.

 
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