Doctor George Logan was a Quaker and deeply moral man. As a Quaker, he was morally opposed to war. Disturbed by President Adams' quasi-war with France, Logan took a trip to France in 1790 at his own expense with an letter of introduction from Thomas Jefferson (the Vice President) to speak to their new government. He met with their brief revolutionary government, particularly with the Maquis de Talleyrand, the French foreign minister at the time. The US was allied with France during the Revolutionary War. If not for France, we never would have won. They were our greatest ally, but when that government was toppled by the French Revolution, the question was ... did we still owe them money because this was a new regime. The US (the Adams administration) said "of course not" and France said "pourquoi pas!" They continued to sink our merchant ships throughout the Caribbean, particularly because we were trading with their enemy Great Britain, and we couldn't stop them because we had no navy at the time. It was this quasi-war that motivated the US to revitalize our navy and create the Marine Corp.
Logan carried with him the message that most Americans were eager for peace. He convinced Talleyrand that the crisis could be overcome by forbearance and he agreed to meet with Adams' delegates if he chose to send them. Logan was even able to come home with released prisoners. All of this he did without the President knowing it. He met with Adams with the message that France was cautious yet would like to meet to discuss a diplomatic resolution. President Adams was intrigue. He seized the opportunity and sent three envoys to meet with France: Eldridge Gerry, Charles Cotesworth Pinckney and John Marshall. The peace agreement that ensued was called the Convention of 1800, also
known as the Treaty of Mortefontaine.
This pissed off quite a few people, those who wanted war. Secretary of War Pickering and Alexander Hamilton, who was merely a very powerful citizen at this point. They wanted to attack France once their navy had been replenished. They had so much influence that they convinced Congress to pass the Logan Act, a law forbidding private citizens from interfering in government business. Only the president had the power to discuss treaties and such. Things haven't changed that much, they mostly passed this law for personal reason because Logan succeeded where they had failed. Logan solved the problem of war. He was never found guilty of anything. The people of Pennsylvania later rewarded him by electing him Senator, but the law still remains.
Should this law even exist? No one has ever been prosecuted under the act, one person indicted and no one ever went to trial. The one person that was indicted, just four years after the bill was written, was a farmer that wrote a newspaper column suggesting that the Western portion of the country become a new country and ally itself with France. It never went to trial. Many have been mentioned in regards to the act, most recently General Michal Flynn, Trump's ex-National Security Adviser, who met with a Russian delegation during the election. If Flynn is indicted, it will most likely not be under the Logan Act.
President Reagan invoked it twice. In 1984, he went after Reverend Jesse Jackson for his trips to Cuba and Nicaragua and then in 1987, when House Speaker Jim Wright interfered in the negotiations between Nicaragua's Sandinistas and the Contra rebels. Threats to other people, like George McGovern and "Hanoi" Jane Fonda, were made but never followed through with.
The most egregious example of someone that should have been prosecuted and jailed over the Logan Act is when presidential candidate Richard Nixon urged his friend, Anna Chennault, to sabotage President Johnson's peace talks between the belligerent parties in the Vietnam War in 1968. Chennault told the South Vietnamese government not to accept the peace deal because when Nixon became president, they'd get a better deal. Nixon wanted the war to end during his administration. If it ended during Johnson's, Nixon might not have won the election. So the war went on for several more years, over 21,000 more people died and when peace finally came, South Vietnam got the same deal that they would have gotten under Johnson earlier. When this came out about Nixon, during his administration, he should have been impeached and both him and Chennault should have faced criminal charges. But we don't impeach over such things in America. We impeach Presidents over blow jobs and hotel break-ins but not for 21,000 people needlessly dying.