Why does Japan's constitution renounce war, and who wrote it?

Because it was written at the end of a war that Japan had lost, under an American-led occupation, and Japanese and American accounts disagree about whose idea the clause was. Article 9 says the Japanese people forever renounce war as a sovereign right of the nation and the threat or use of force as a means of settling international disputes, and that land, sea and air forces will never be maintained.

Japan surrendered on 2 September 1945 and was occupied by the Allies until April 1952. The US general Douglas MacArthur oversaw it as Supreme Commander for the Allied Powers. MacArthur resisted calls to try Emperor Hirohito as a war criminal, arguing that a prosecution would be overwhelmingly unpopular, and, at the occupation's insistence, the emperor publicly renounced his own divinity.

A new constitution replaced the Meiji Constitution of 1889. A Japanese committee under Jōji Matsumoto first proposed only cautious changes, which MacArthur rejected outright, ordering his own staff to draft a new document. The result was mostly drafted by American authors and then reviewed and modified by a few Japanese scholars; Japan's lower house added wording to Article 9 credited to Hitoshi Ashida. It transferred sovereignty from the emperor to the people and took effect on 3 May 1947.

Who first suggested Article 9 is disputed. MacArthur, speaking when the US wanted Japan to rearm, said Prime Minister Kijūrō Shidehara proposed it. The constitutional scholar Toshiyoshi Miyazawa argued it came from MacArthur himself. Our main source says the most recent research supports Shidehara's authorship.

The clause met reality quickly. When the Korean War began in 1950, the US pulled a division out of Japan, and MacArthur ordered a 75,000-strong National Police Reserve. To avoid constitutional trouble, tanks were called "special vehicles". In 1954 the reserve became the Japan Self-Defense Forces.