His indictment was a watershed moment in the effort by the United States to pierce the veil of Swiss bank secrecy: The United States alleged UBS' so-called "cross border" business had approximately 20,000 American clients with assets hidden from the IRS totaling as much as $20 billion. In 2009, UBS agreed to pay a $780 million fine and turn over the names of thousands of clients.
Weil was CEO of a division of UBS that oversaw the American cross-border business and worldwide private banking. His lawyer has said he is not guilty, and Weil intends to defend himself at trial. Because of Weil's senior role, investigators have surmised for years that he has detailed information on potentially illegal activities by some of America's wealthiest families—names that have never before been made public.
If convicted, Weil faces up to five years in prison and a $250,000 fine. In May, a former UBS banker, Martin Lack, was sentenced to five years' probation after pleading guilty to conspiracy.
The Weil trial could reveal new details about the scope of UBS' fraud and the steps the bank took to conceal it, said veteran financial crime expert Jack Blum. "The government is anxious to pursue it," Blum said. "They want to send a message to others. If you are in Singapore doing this stuff now, watch out."
Read MoreReport: UBS faces $6 billion fine in France
What will probably not be revealed, Blum said, are the identities of the thousands of Americans who had secret accounts at UBS to cheat on their taxes. "The public can't know those names," Blum said. "I think it's wrong. I think the public should know—if people were evading taxes, it should be a matter of public record, and people should be embarrassed by it."
But already, the legal maneuvering surrounding Weil's case has revealed new details of the high-level behind-the-scenes diplomatic negotiations over UBS in 2008 and 2009.
Documents filed in the case show Swiss regulators met with or lobbied the chairman of the Federal Reserve, Treasury secretary and U.S. attorney general in an effort to blunt the U.S. government's pursuit of the bank's American clients.
One document is a 2010 report by the Swiss Financial Market Supervisory Authority (FINMA) that was translated from the original German for the Weil trial. The report refers to a Jan. 12, 2009, meeting between then-Fed Chairman Ben Bernanke, the president of FINMA and the president and vice president of the Swiss National Bank. "The chairman of the US Fed reconfirmed the threat at this meeting and pointed out the [Department of Justice's] independence to act on the threatened criminal prosecution," the report said. "The Federal Reserve Bank, according to this information, had no right of veto."
A representative of the Federal Reserve declined to comment on the alleged Bernanke meeting with the Swiss.
Read MoreUS uses new tactic to crack laundering cases
Swiss authorities also attempted to persuade then-Attorney General Michael Mukasey and Treasury Secretary Hank Paulson to "refrain from unilateral measures against UBS to request client information located in Switzerland." The Swiss officials wrote in a letter dated Nov. 10, 2008—at the height of the global financial meltdown—that they had "taken unprecedented measures … in order to stabilize financial markets." An aggressive pursuit of UBS by the United States, they warned, "could put at risk the effectiveness of these measures."
Informed of the new documents, Sen. Carl Levin, D-Mich., chairman of the Senate Permanent Subcommittee on Investigations, said "it is not surprising that senior Swiss government officials sought to derail U.S. efforts to enforce our tax laws." He added: "As our investigations have shown, a key factor in the use of Swiss banks to evade U.S. taxes has been the Swiss government's efforts to protect Swiss banks from accountability."
The government's star witness in the trial is expected to be Martin Liechti, who was once UBS' top private banker for the Americas. Liechti secured a nonprosecution agreement with the Justice Department in July 2008 in exchange for his cooperation in the wider investigation. Liechti was required as part of that agreement to turn over the names of UBS clients "who had or have undeclared accounts with UBS AG."