The Nanking Defence War and Dunkirk were also surrounded on three sides, and the last resistance was retreating. The difference is that one crosses the river and the other crosses the sea.

1 there is a big gap in naval strength, and the British army is the number one naval power. At that time, the Kuomintang navy was basically sunk, and it was definitely impossible to retreat from the river without a ship. As we all know, corruption and factional groups of the Kuomintang often occurred in the Nanjing Defence War, and the troops were defeated without orders. Chiang Kai-shek was ordered to stick to it. Later, the British army ordered to retreat and actively rescued. The Japanese army tried its best to win Nanking, and basically had no time to organize evacuation, while the Germans in Dunkirk stopped advancing for several days. Of course, the weapons of China's army were far inferior to those of the Japanese army, resulting in a total rout, while the British-French allied forces were almost the same as the Germans. If you really want to defend it, it is not easy to stop it temporarily, which is conducive to the evacuation of the organization.