On the 3rd of September 1939, France and Great Britain declared war on Germany. The war started with a quiet period of 8 months, with only a few minor incidents between the two sides. Germany did not immediately launch an attack because they first tried to establish peace again with Britain and France.
This so-called "Phoney war" came to an end when Germany invaded the Netherlands, Luxembourg, and Belgium on the 10th of May 1940 and soon after this 'blitzkrieg" attack, the Battle of France began.
The Battle of France
The German forces moved through the forest in the Ardennes region, along the Somme valley to the English Channel. With this move, they bypassed most of the defensive structures that France had built to prevent a German attack and they succeeded to flank the allied forces.
Surprisingly, at the crucial moment that the German troops could obliterate the allied forces, Adolf Hitler gave the order to delay the advancement of the troops. Whether it was out of fear of counterattacks or out of overconfidence, we can't really tell for sure, but this would turn out to be one of the crucial mistakes the German dictator had made that could have completely altered the outcome of the war. Thanks to this hesitation, the Allied troops had time to prepare for a mass evacuation.
The Miracle of Dunkirk
In the evening of the 26th of May 1940, the Allied forces started the evacuation of their troops from Dunkirk under the code-name Operation Dynamo. On this first day, around 7500 men escaped.
On the 27th May, the Luftwaffe started to bomb Dunkirk, endangering the evacuation. The British Royal Air Force fought back and around 18000 men got evacuated on that second day.
Starting on the 28th of May, small vessels came to the aid of the military boats. The shallow sandy beaches of Dunkirk prevented larger ships from reaching the coast, so efforts were made to search small boats that could easily get close enough. This included speedboats, car ferries, Thames vessels, pleasure craft, ... Many of these boats were steered by civilian crews, as the military lacked the necessary personnel. In total there were over 800 boats that aided in the evacuation.
At the end of Operation Dynamo around 338000 men were rescued and 40000 French troops that stayed behind were forced to surrender to Germany. The British Army also lost a great deal of equipment that they left behind on the beaches.
Afterword
If you read the story of the Battle of Dunkirk, you can see that this was a great loss for Britain. However, if the evacuation had failed, Britain would have had to agree to peace with Germany. Instead, they continued in the war and prime minister Winston Churchill even used the events that happened in Dunkirk as a method to invoke pride and patriotism in the hearts of the British public:
"... Even though large tracts of Europe and many old and famous States have fallen or may fall into the grip of the Gestapo and all the odious apparatus of Nazi rule, we shall not flag or fail. We shall go on to the end, we shall fight in France, we shall fight on the seas and oceans, we shall fight with growing confidence and growing strength in the air, we shall defend our Island, whatever the cost may be, we shall fight on the beaches, we shall fight on the landing grounds, we shall fight in the fields and in the streets, we shall fight in the hills; we shall never surrender ..." - Winston Churchill