Britain and France did not declare war on the USSR for a simple reason: they considered it more difficult to go to war with the USSR than with Germany. First of all, Germany is small in size and lacks resources, and once it is blocked, it will face supply difficulties, while the Soviet Union is a vast country with abundant resources, and even if it is blocked, it can be self-sufficient, and its vast coastline makes the blockade even more difficult.
Secondly, Germany was narrow and its strategic depth was insufficient, and Britain was conducive to bombing Germany, while the Soviet Union's strategic depth and remote inland location made it difficult for bombers at that time to reach their targets. In addition, the German climate was suitable for the Anglo-French forces to adapt to, while the Soviet climate was harsh, and the winter landing operation was almost doomed, especially the French did not want to repeat the mistakes of Napoleon.
Therefore, the choice of Britain and France not to go to war with the Soviet Union was a logical decision. In short, the combined forces of Britain and France were unable to defeat the Soviet Union, and although the Soviet Union was slightly inferior to Germany militarily, its geographical conditions made it an easy territory to defend and difficult to attack. The war against the Soviet Union would cost enormous human, material and financial resources, far exceeding the consumption of Germany.
If the British and French forces declared war on the Soviet Union, it would be costly, costly, difficult to win, and possibly even cause civil unrest. The foreign policy of the British and French coalition also illustrates their prudence, claiming to help the weak and the strong, but not daring to send troops against **, because ** is too strong.
In addition, according to international law, the mutual defense treaty they signed was basically directed against Germany, and they did not even impose sanctions on the Soviet Union because they did not expect the Soviet Union to take aggressive actions.
Britain and France considered going to war with the Soviet Union, but when they saw that Nazi Germany had invaded Norway, the Netherlands, Belgium, France, Denmark, and Greece, they did not even dare to mention the idea again. Eventually, they had hoped to shift the war to the East, but they did not expect their country to be attacked first.