Soviets declare war on Japan; invade Manchuria

On August 8, 1945, the USSR declared war against Japan in WWII. How opportunistic is that? From the article:

"Soviets declare war on Japan; invade Manchuria
On August 8, 1945, the Soviet Union officially declares war on Japan, pouring more than 1 million Soviet soldiers into Japanese-occupied Manchuria, northeastern China, to take on the 700,000-strong Japanese army.

The dropping of the bomb on Hiroshima by the Americans did not have the effect intended: unconditional surrender by Japan. Half of the Japanese inner Cabinet, called the Supreme War Direction Council, refused to surrender unless guarantees about Japan’s future were given by the Allies, especially regarding the position of the emperor, Hirohito. The only Japanese civilians who even knew what happened at Hiroshima were either dead or suffering terribly.

Japan had not been too worried about the Soviet Union, so busy with the Germans on the Eastern front. The Japanese army went so far as to believe that they would not have to engage a Soviet attack until spring 1946. But the Soviets surprised them with their invasion of Manchuria, an assault so strong (of the 850 Japanese soldiers engaged at Pingyanchen, 650 were killed or wounded within the first two days of fighting) that Emperor Hirohito began to plead with his War Council to reconsider surrender. The recalcitrant members began to waver."