Do you think that the evacuation of Dunkirk in 1940 was a succes for the British and why?
I personally think that it was a terrific success for the allied forces (British, French) who evacuated their main forces (330, 000 soldiers) in 9 days, but what's your view?
It was a successful retreat. But did the Germans let them get away? This does seem consistent with the Germans curious strategy involving Britain throughout the war.
The evacuation was, of course, a "success," but that follows only on the disaster of the failed British Invasion. Almost all of Britain's armor was lost along with a goodly amount of their other war materiel. Of course, getting the soldiers back to Britain was key, and as others have pointed out, they came back in force a few years later to help the Germans relocate. But to call the evacuation a success in the aftermath of the failed invasion is not quite accurate.
The evacuation itself was a remarkable success in that it preserved several divisions of the British Army. It was also a remarkable blunder on the part of German generals, who decided to try to soften up the encircled British and French forces with aerial bombardment instead of proceeding with a direct assault with armor. This allowed the British time to mobilize an evacuation flotilla.
Of course the evacuation was a success - the troops were not captured or killed by the enemy. As post 3 says, the reason the evacuation was needed was because of the failure of the Allied forces to stop the advance of the Germans as they advanced toward France. However, your question just asks about the evacuation, not the reasons for it.
I don't see any way that you could say that this was not a success. Imagine if essentially the whole British army had been captured in 1940. It seems very likely that an invasion of Britain would have been possible (though I know there was still the RAF and RN to deal with) and the British people would have been utterly demoralized. If they managed to survive, that is still, as Post 3 says, 300,000+ men who would not have been available to fight come D-Day and beyond. This was a huge "victory" for the Allies.
The successful evacuation of Allied troops boosted the British morale alright, but it ended a wholly unsuccessful campaign in which the Germans separated the remaining French and Belgian armies, cutting them off from helping the troops who had retreated to Dunkirk. It left the French and Belgians alone to fight the Germans, and they were soon forced to surrender. It should also be pointed out that the German decision to halt their offensive for three days allowed the British time to evacuate. This decision was criticized at the time and has been debated for decades afterward. Had the Germans pushed forward, they would have probably isolated the entire Allied force on the beaches and forced their surrender. The German decision to halt was to make certain that the French and Belgians would not be able to break through and meet up with the isolated Allied Forces. In the end, it gave the Allies an additional 300,000+ men to fight again, but it also gave up the European mainland to full control of the Germans.