With the amazing strength of the German War machine, overrunning all in its way, they managed to encircle the British Expeditionary Forces and a large number of French soldiers.
The Allies had no idea what was awaiting them and they were ill prepared against the Nazi's tanks, bombers and other superior weaponry. They had to be evacuated immediately, as they were taking enormous losses of their soldiers.
Britain had to evacuate more than 100.000 troops including British, French and some Poles who managed to get to the evacuation points. On May 26th till June 4th the evacuation lasted with Nazi's fighter planes and bombers in action. They even tried to play strafe the soldiers in hiding places. The British Hurricanes and Spitfires helped to make a success of "Operation Dynamo" as the evacuation was called.
One problem was to get together enough ships to do the job. There wasn't much of a harbour, hence war ships couldn't get close enough. This and relentless German bombing caused an immense problem. The soldiers were so eager to get to safety, causing chaos.
It was at this point that Churchill amassed all seagoing vessels. Such as barges, holiday boats and that which could float. Eventually 860 crafts sailed across the channel. Although many vessels succumbed because of bombing and some capsized as a result of anxious men trying to be saved, 132000 British were safe on English soil, as well as +, - 21000 Frenchman arriving safely.
The were disastrous losses of men, many injured and sea vessels. The irony I see in this is that so many had to be evacuated, but 2 years later the Allies had to think and reason hard to get man and machine back there. This would of course be " D - Day".
Sources: Stories of WW2 ( Readers Digest) A compilation of WW1 and 2. Google Images