Why did the Soviets invade Manchuria on the last week of WWII?

I've always thought it was a bit strange how the USSR declared war on Japan on the day when the US dropped the nuke on Hiroshima. It's said that the idea was to gain concessions, and they certainly gained concessions. But strategically it makes no sense. And I don't think the US ever believed in the importance of the Soviet contribution to the war in the Pacific because the US invaded Hiroshima after the Soviets had declared war. <br><br>So what really was the deal?<br><br>Addendum: It looks like the answer is in the comments below. But for anybody who wants a summary, it's this: <br><br>The Soviet Union had promised to join the war after a three month window following Germany's surrender, which happened on May 8, 1945. They kept their promise. <br><br>The Americans dropped the bomb on Hiroshima to demonstrate the destructive power of the bomb to the Soviet Union. They then went forward with a second nuclear bomb attack on Japan, which prompted the Japanese surrender on August 15, 1945.<br><br>So to recap: The Soviet Union declared war on Japan because they had promised to do so, and they kept their word. <br><br>The Americans dropped two nuclear bombs on Japan because they wanted to demonstrate the power of the bomb to the Soviet Union. The Japanese surrendered before the Soviets had time to conquer any Japanese territory. <br><br>So the Americans use nuclear weapons to demonstrate their power to the Soviets, and the Soviets honour their promise to declare war on Japan.