Was the USSR ready for the Second World War and why? could a world war have been prevented?

Was the USSR ready for the Second World War and why? could a world war have been prevented? Was it necessary to surrender Leningrad? Could the USSR have won a victory without allies in World War II and Patriotic War?

In theory, the USSR was ready to attack. The troops received new weapons, the units were stationed on the border. In practice, the Nazis had a combat-ready and victorious army, equipped with modern weapons. In addition, the Soviet leadership did not attach importance to the reports of the spies.

It is possible, if it was possible to promptly suppress the aggression of Germany and its allies, preventing them from conquering Europe.

No, since Leningrad was not only one of the largest cities, but also an industrial center. It was a symbol of the revolution.

In theory, yes, but it would be worth even more losses. Allied help was very important.