If Michael Flynn, President Trump's former national security adviser, was trying to reassure the Russian ambassador about sanctions leveled by the Obama administration, then he may have broken the law.That law is the Logan Act. It's gotten a lot of attention recently and is at the center of the scandal surrounding Flynn, who resigned Monday night after multiple reports were published noting that he had misled the vice president and others in the administration. Flynn categorically denied to Mike Pence, then vice president-elect, last month that he had discussed the sanctions.That has turned out to be not true. And Pence unwittingly repeated the falsehood on Sunday shows. White House press secretary Sean Spicer said Tuesday that Trump asked for Flynn's resignation because of an "eroding level of trust." He reiterated that it was not a "legal issue, but a trust issue.""There was nothing wrong or inappropriate about those discussions," Spicer contended.But without the release of the transcript of the phone call — which exists — between Flynn and the Russian ambassador, there's no way to know if there isn't a legal issue.So what is the Logan Act, has it ever been prosecuted and could Flynn be susceptible?First, what is it? The Logan Act states, in part: "Any citizen of the United States, wherever he may be, who, without authority of the United States, directly or indirectly commences or carries on any correspondence or intercourse with any foreign government or any officer or agent thereof, with intent to influence the measures or conduct of any foreign government or of any officer or agent thereof, in relation to any disputes or controversies with the United States, or to defeat the measures of the United States, shall be fined under this title or imprisoned not more than three years, or both."Let's break that down. This is saying that no private person can try to conduct foreign policy without the permission of the U.S. government. Specifically, that person isn't allowed to talk with a foreign government or its representative and try to influence foreign policy.When was it put in place and why?It's named for Dr. George Logan, a Republican lawmaker from Pennsylvania, who traveled to France in 1798 and negotiated a lifting of an embargo and the release of American sailors held captive in French prisons. He did so without the permission of President John Adams, though he was a friend of then-Vice President Thomas Jefferson.It was after the French Revolution, and the United States didn't help France because of a treaty with England. The French were upset and began robbing passing American ships and jailing its sailors. Official Adams administration envoys tried unsuccessfully to settle the unrest with France — and the country appeared tumbling toward war with France.That's when Logan, a Quaker, embarked on his effort. He succeeded, but when he got back to the U.S., Adams, Washington and others were furious. Congress then acted to bar actions like Logan's from taking place again.Has anyone ever been prosecuted?No, but plenty of people have been mentioned in relation to it and at least one was indicted.The indictment was way back in 1803, four years after the law was put in place. An anonymous column ran in a Kentucky newspaper arguing for "A Western America" allied with France. The U.S. attorney for Kentucky was an Adams ally and got a grand jury to indict Francis Flournoy, a farmer, who actually wrote the column. But it never went to trial. Later that year, the Louisiana Purchase, which expanded the U.S. West, made it moot.Various courts have mentioned the Logan Act, as the Congressional Research Service points out, but it has mostly been used as a weapon to try to shame political opponents. For example, in the 1980s, there was former House Speaker Jim Wright's relationship with the Sandinista government in Nicaragua and Jesse Jackson negotiating in Syria and Cuba for prisoner releases."That is the law of the land," a frustrated President Ronald Reagan said in an interview in 1984 of the Logan Act and Jackson's efforts, adding, "We are not going to take legal action, but I do feel that while in this instance he was successful, there were things that make you pause and think."During the George W. Bush administration, House Speaker Nancy Pelosi took a trip to Syria and talked about improving U.S.-Syria relations with that government, irritating the White House. Former President Jimmy Carter also traveled to Syria in 2008 to meet with Hamas about Mideast peace. As recently as 2015, the act was mentioned again when 47 Republican U.S. senators wrote a letter to Iran downplaying the nuclear deal.And Syria raised the potential of a Logan Act violationwhen Democratic Rep. Tulsi Gabbard of Hawaii met with Syrian leader Bashar Assad last month before President Trump took office."I wanted to see if there was, in some small way, a way that I could express the love and the aloha and the care that the American people have for the people of Syria," she told CNN.The Obama administration's official position was regime change in Syria.Could Flynn be susceptible?If Flynn, in any way, reassured the Russian ambassador in his phone conversations or texts that the incoming Trump administration would loosen or drop the sanctions Obama put in place, that could potentially be a violation.As noted above, there is a transcript. The White House is confident Flynn didn't say anything that could have violated the law. Of course, that could be open to interpretation. It would be up to the Jeff Sessions Justice Department to review the transcript and bring a case, if it saw fit. That is seen as unlikely.But there are other ways in which Flynn could be at risk.Republicans on the Senate Intelligence Committee said Tuesday it's possible that Flynn will be called to testify under oath. More could become known about the specifics of the call before then. The New York Times reports Tuesday evening that the FBI questioned Flynn in the early days of the Trump presidency about his conversations. And investigators believe Flynn "was not entirely forthcoming":"That raises the stakes of what so far has been a political scandal that cost Mr. Flynn his job. If the authorities conclude that Mr. Flynn knowingly lied to the F.B.I., it could expose him to a felony charge." Copyright 2017 NPR. To see more, visit http://www.npr.org/.