Harry Truman was not well-prepared to lead the nation in terms of the amount of time that he had been in high office and the degree to which he was in the know on important aspects of the war effort. Truman had, of course, not been Roosevelt's vice president for his first three terms. Therefore, he had only been vice president for a few months when he became president. Because he had not been in office very long at all, he did not even know of the existence of the Manhattan Project and the atomic bomb.
However, it seems clear that Truman was prepared to lead the country from the standpoint of his personal qualities. Though he was thrown into office abruptly and without proper briefings, he quickly got control of the situation. He was able to manage very well with the decisions that had to be made at the end of the war, not least of which was the decision to drop the atomic bomb. Overall, then, Truman was a man who had the personal qualities that allowed him to be ready for a job that he had not been well prepared for.