SoftBank is making a bold move in artificial intelligence (AI), not only as an investor in the $500 billion Project Stargate—a major initiative to expand AI infrastructure in the US but also through a new Japanese joint venture with OpenAI and a $3 billion deployment of OpenAI’s technology across its businesses.

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,