If completed, the proposed $20 billion takeover of Sprint Nextel Corporation by Tokyo-based Internet and telecommunications group SoftBank Corp. will be the largest U.S. acquisition by a Japanese company ever.

No wonder, then, that SoftBank counsel Morrison & Foerster had as many as 16 partners working around the clock to get it done, with Tokyo managing partner Kenneth Siegel leading a team in Japan and global mergers and acquisitions head Robert Townsend and his staff working from San Francisco.

“We basically just executed 24 hours a day,” says Siegel.

It was no huge surprise to the market that Morrison & Foerster was at the wheel. The firm has one of the largest and most successful Japan practices of any international firm, with 125 lawyers in its Tokyo office. With a huge boom in Japanese overseas investment now underway, Morrison & Foerster is in the sweet spot. The SoftBank-Sprint deal brings the value of the firm’s Japan M&A deals to a whopping $35 billion so far this year.

Siegel and Townsend are both old Japan hands. Townsend served as managing partner of Morrison & Foerster’s Tokyo office from 1993 to 1996. Siegel, who helped launch the office as an associate and learned to speak flawless Japanese after he met the Japanese exchange student he eventually married, has led it ever since. The firm’s biggest practices in Japan are M&A and intellectual property litigation, and its stable of loyal clients there includes household names like Toshiba Corporation, Hitachi, and Fujitsu. (Last year our sibling publication The Am Law Daily named Siegel a Dealmaker of the Week for advising Hitachi on the sale of its hard-disk business to Western Digital Corporation.)

SoftBank joined that client roster during the 1990s dot-com boom, when chief excutive officer Masayoshi Son launched Yahoo! Japan and the Japanese subsidiaries of E*Trade and E-Loan, although the firm declined to state its involvement in any specific transaction from that period for confidentiality reasons. MoFo was also there when SoftBank expanded into the mobile business by buying Vodafone Japan for $15 billion in 2006. Earlier this year, Morrison & Foerster advised the company yet again on the $7.6 billion Alibaba-Yahoo share buyback that made SoftBank the Chinese Web portal’s largest shareholder. Siegel and the firm also guided the company on its $2.3 billion equisition of domestic rival eAccess earlier this month.

And now the firm is helping Son realize his dream of entering the U.S. market. Siegel says that the CEO is a hands-on kind of client.

“It was an aggressive schedule and a complex transaction structure, but we’re very glad to have had Mr. Son to push the process along and get the deal done,” Siegel says.

The deal with Sprint, which is being represented by Skadden, Arps, Slate, Meagher & Flom, still needs regulatory approval. Siegel says he’s not worried about any antitrust issues holding up the deal, noting that other U.S. mobile companies have some foreign ownership. Verizon Wireless is a joint venture between American telecom company Verizon Communications Inc. and British carrier Vodafone Group plc, while T-Mobile USA, Inc. is backed by Germany’s Deutsche Telekom AG.

Email: tbrennan@alm.com