There is a fundamental difference between the two—a difference that meant we knew we could probably build a fission bomb, and, later, fission reactors, but that controlled fusion is and will always be impractical. Nuclear fusion happens spontaneously in nature. The problem is keeping it from happening, and controlling the process. During the Manhattan project there were tragic events were fisson happened accidentally. Fusion is different because you must actively maintain conditions that nature is trying to disrupt.