In his newly built palace near Tokyo, lined by stone statues of Roman emperors and surrounded by an 18-hole golf course, Masayoshi Son was stewing. After declaring for years the imminent arrival of the artificial-intelligence revolution,

Masayoshi Son, the billionaire founder and CEO of SoftBank, the Japanese media technology conglomerate, is often cast as a dreamer, financial engineer, and speculator. But his career — which has spanned the launch of the personal computer and internet,