The order to stop the tanks outside Dunkirk threw away a complete victory over Britain: the whole British military was encircled and got out.

So the tank generals received the Halte Befehl, the stopping order. They didn't believe it when they received it because this would have been a complete victory over Great Britain. This would have been the end of Great Britain. The whole British military was encircled, but they did get out through Dunkirk.