SoftBank owns 80 percent of Sprint, which is perhaps Mr. Son’s marquee American investment, but that control would be relinquished under the terms of the merger announced Sunday. SoftBank would own just 27 percent of the combined company, which would be named T-Mobile, and have four of the 14 seats on the company’s board of directors. (T-Mobile’s current parent company, Germany’s Deutsche Telekom, would own 42 percent of the new company and hold nine seats.)
Even with an agreement in place, the deal is not done. Regulators in Washington will scrutinize the merger, which would cede control of the United States wireless market to just three carriers. In the past, attempted mergers in the industry have been rejected on the grounds that more competitors are better for consumers because they result in lower prices and superior services.
Regardless of the outcome, the agreement is the latest in a series of bold wagers that have defined Mr. Son’s investing career.
In 2000, he put $20 million into Alibaba, the Chinese internet company. As a result of that investment, Mr. Son now owns 30 percent of the $440 billion shopping giant.
Mr. Son and SoftBank became serial acquirers. The company built up positions in the Japanese mobile phone market, including by purchasing the British semiconductor designer ARM Holdings, which develops critical technologies for the mobile market.
More recently, Mr. Son has focused on technology companies in the United States. The vehicle for those efforts will be Mr. Son’s London-based Vision Fund. While the fund represents the vision of Mr. Son, it is bankrolled by Saudi Arabia, which has put up $45 billion — nearly half the fund’s size.
While some have hailed Mr. Son as the Warren Buffett of technology investing, that comparison misses the mark. Mr. Buffett is a forensic analyst of companies rather than trends, and he usually opts for what he perceives to be strong, undervalued brands rather than putting his money into fast-growing, innovative — and risky — firms.
Mr. Son is more entrepreneur than investor in the classic style.
As a university student, he invented a language translator that he sold to a computer company. As an investor, his forte has been identifying futuristic themes and investing based on them. He now is predicting that interconnected devices and artificial intelligence will be the next big trends to disrupt life and industry.
Unlike Mr. Buffett, Mr. Son, 60, relies on an army of hundreds of investment bankers and analysts. They travel the world, identifying potential corporate targets and then structuring deals to acquire them.
But such thematic investing can backfire. Many of Mr. Son’s technology bets fell victim to the 2000 technology crash. And skeptics say the enormous Vision Fund could stumble into bad, overpriced deals as it tries to deploy tens of billions of dollars in a short period of time. One such eyebrow-raising deal was a $300 million investment in Wag, a dog-walking service built around a smartphone app.
Mr. Son also faced criticism for his purchase of 80 percent of Sprint, given the company’s size, shaky finances and the difficulties of winning American regulators’ blessings for any mergers.
Now, Mr. Son is calculating that under President Trump, the Federal Communications Commission will look more favorably on such a merger than it did during the Obama administration.
It is no sure thing — the Trump administration is suing to block a proposed merger between AT&T and Time Warner, citing the size of a combined entity.
While not known for his political instincts, Mr. Son was savvy enough to get a meeting with Mr. Trump in the early days after the election.
The president-elect publicly praised Mr. Son’s promise to invest $50 billion in the United States and create 50,000 jobs.
“He is one of the great men of industry,” Mr. Trump said to reporters in the lobby of Trump Tower in December 2016.
The question was then put to Mr. Son: Why had he come to meet Mr. Trump so soon after the election?
“I came to celebrate the president’s new job,” Mr. Son said. And in their meeting, he said, Mr. Trump told him “that he would do a lot of deregulation.”
That was music to Mr. Son’s ears, given his ambition to secure a merger.
“I said this is great,” Mr. Son said. “The United States would be great again.”