Why did the Soviet Far East Army kill the Kwantung Army, the flower of Japan's strongest army, within a week?

Japan's invasion of China is scattered, and the Japanese Kwantung Army is only a part of Japan's military forces deployed in the northeast. After years of war consumption, Japan is already a spent force, and most of the Japanese army is contained by the China army. After the Soviet army defeated the Germans, the troops quickly retreated and suddenly declared war on Japan with absolute military strength (Katyusha rockets shocked the enemy with the sound of soul-urging in World War II), which made the Japanese army unprepared and disrupted the overall deployment. Of course, it is also doomed to failure.