Answers May vary, but your response should include the following points: Stalin promised at the Yalta conference that he would enter the war in the Pacific once Germany had surrendered in Europe. Although he kept his promise, the United States didn’t seem to need his help after the bomb was dropped on Japan. Stalin rushes to establish a Soviet but hold on Manchuria and Korea. Having join the war against Japan, the Soviets expect to have a say in how Japan’s holding would be divided up among the victorious allies.
At the Yalta Conference in February, 1945, Stalin had agreed to enter the war against Japan three months after Germany was defeated. Victory in Europe was achieved on May 8, 1945. The Soviet Union declared war on Japan on August 8, 1945, and invaded Manchuria with over a million troops to take on the Japanese army there.
As to the dropping of the second atomic bomb, even the dropping of the first could be challenged when factoring in the USSR. An option to dropping atomic bombs was to enlist Soviet troops in a joint invasion of Japan. But the USA wanted to avoid postwar Soviet presence in Japan, and the atomic bombs were seen as a way of ending the war quickly. As to the use of a second bomb at Nagasaki after the first was dropped on Hiroshima, it was because of the Allies' requirement that Japan submit to an unconditional surrender. They did not do so in the immediate aftermath of the Hiroshima bombing, so the second bomb was used. You can consider for yourself whether some other resolution besides "unconditional surrender" was a viable option.
The U.S. entered World War I because Germany embarked on a deadly gamble. Germany sank many American merchant ships around the British Isles which prompted the American entry into the war.
