T-Mobile-Sprint Merger Would Give Japan’s SoftBank Bigger Foothold in U.S.

Masayoshi Son, the founder of SoftBank, in February. Since SoftBank acquired Sprint in 2012, Mr. Son has vowed to take on the American market leaders Verizon and AT&T.

In an era of increasing foreign investment in the United States, few have been as acquisitive — or disruptive — as Masayoshi Son, the billionaire Japanese founder of SoftBank.

With his $98 billion Vision Fund, he has barged into Silicon Valley, paying top dollar for big stakes in the ride-sharing giant Uber and the office-space company WeWork. He has also muscled onto Wall Street’s turf with his launch of a private equity firm that hopes to rival the titans Blackstone and K.K.R.

In the United States, his highest profile wager has been his majority ownership of Sprint, the nation’s fourth-largest wireless provider.

Ever since that 2012 acquisition, Mr. Son has vowed to take on AT&T and Verizon, the leading United States mobile companies. But Sprint, burdened with a heavy load of debt, has struggled, and the goal remained elusive. Now Mr. Son is making another gamble.

On Sunday, Sprint and T-Mobile, the fast-growing telecommunications firm that ranks ahead of Sprint in terms of market share, agreed to merge. The transaction would create a giant with about 100 million customers that will be able to go toe-to-toe with AT&T and Verizon in the battle to dominate the next frontiers of wireless technology.

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.