Context:
The question is asking which country the United States knew was about to surrender during a specific period. To answer this question, we need to examine the historical context and events leading up to the surrender of a particular country.

Explanation:
During World War II, the United States was involved in fighting against several countries, including Japan, Germany, and Italy. However, the surrender mentioned in the question is most likely referring to the surrender of Japan.

The Surrender of Japan:
Japan was one of the Axis powers during World War II and had been engaged in conflict with the United States in the Pacific theater. The war in the Pacific had been particularly brutal, involving intense battles, bombings, and significant loss of life.

The Atomic Bombings:
In an effort to force Japan to surrender and bring an end to the war, the United States made the decision to drop atomic bombs on the Japanese cities of Hiroshima and Nagasaki in August 1945. These bombings caused immense destruction and loss of life, leading to a significant impact on Japan's military capabilities and civilian population.

The Potsdam Declaration:
Following the atomic bombings, the United States, along with the United Kingdom and China, issued the Potsdam Declaration on July 26, 1945. This declaration called for the unconditional surrender of Japan and outlined the terms under which Japan would be governed after surrender. The declaration warned of "prompt and utter destruction" if Japan did not comply.

The Surrender:
Despite initially rejecting the Potsdam Declaration and vowing to fight on, Japan faced immense pressure and realized the severity of the situation. On August 15, 1945, Emperor Hirohito announced Japan's surrender in a radio broadcast. This marked the end of World War II in the Pacific and the beginning of the occupation and reconstruction of Japan by the Allied powers.

Conclusion:
In conclusion, the United States knew that Japan was about to surrender during World War II. The surrender came after the atomic bombings of Hiroshima and Nagasaki and the issuance of the Potsdam Declaration, which demanded Japan's unconditional surrender. The surrender of Japan ultimately brought an end to the war in the Pacific and set the stage for the post-war reconstruction and occupation of Japan.