In the early twentieth century, adventurous Japanese businessmen, diplomats, and military officers produced on-the-ground information that helped Japan defeat Russia and invade China. But Japanese intelligence gathering went into decline thereafter. Military domination of intelligence work fostered groupthink, which led to spectacular mistakes, such as underestimating the U.S. response to the attack on Pearl Harbor. After World War II, Japan’s intelligence agencies suffered from weak public support, turf battles, a failure to share information, and constant leaking. With the end of the Cold War, the rise of China, the growing threat from North Korea, and the relative decline of U.S. power, a series of Japanese prime ministers started strengthening the system. They tightened classification rules, invested in cybersecurity, and established the Defense Intelligence Headquarters and, later, the National Security Council to improve communication among agencies. This engrossing history of Japanese intelligence demonstrates how such changes have made Japan a better security partner for the United States while preparing the country to stand on its own if the U.S. security guarantee loses its credibility.