Why was the United States extremely eager to secure the Soviet Union's participation in the projected invasion of Japan in early 1945?

The United States was extremely eager to secure the Soviet Union's participation in the projected invasion of Japan in early 1945 because it wanted to maximize the pressure on Japan to bring about a swift capitulation and thus avoid the immense casualties of a prolonged land invasion. Knowing that a two-front land offensive would be too costly for Japan, the United States wanted to bring the Soviet Union into the war as soon as possible. This would force them to increase the total number of troops available for the invasion, allowing for a larger-scale campaign. It would also reduce the burden on the United States, which was shouldering a large portion of the effort and costs associated with the war. In addition, having the Soviets join the war would also present a psychological message of a “united front” against the Japanese and make them more inclined to surrender.