Truman asked Roosevelt's cabinet members to remain in their positions, telling them he was open to their advice. He emphasized a central principle of his administration: he would be the one making the decisions, and they were to support him. During World War II, with the invasion of Japan imminent, he approved the schedule for dropping two atomic bombs to avoid a costly invasion of the Japanese mainland. Military estimates for the invasion of Japan were that it could take a year and result in 250,000 to 500,000 U.S. casualties. The U.S. bombed Hiroshima on August 6, and Nagasaki three days later, leaving 105,000 dead. The Soviet Union declared war on Japan on August 9 and invaded Manchuria. Japan agreed to surrender the following day. Truman said attacking Japan with atomic bombs saved many lives on both sides. With the end of World War II, Truman implemented the Marshall Plan. Apart from primaries and campaigning in 1948, Truman dealt with the Berlin Blockade, which is considered the first major diplomatic crisis of the Cold War. During his presidency, Truman's approval ratings had dropped significantly from 80% in early 1945 to 30% in early 1947. The 1946 mid-term election alarmed Truman when Republicans won control of both the houses of Congress for the first time since the 1920s. In 1947, Truman told his Secretary of Defense James Forrestal that, except for the reward of service, he had found little satisfaction in being President.