The assassination of US President James Garfield. It's probably the least well-known of the four presidential assassinations, but it looks like the most likely to have involved a conspiracy.
Garfield was pretty well-liked, and had a reputation for fighting corruption and speaking out against segregation. Garfield's Vice President, Chester A. Arthur, was a puppet of corrupt New York Stalwart Republican politician Roscoe Conkling. To make a long story short, Conkling helped Garfield get elected, wasn't given the lucractive position he felt owed (US Customs Collector), spent the next few years scheming against Garfield and ultimately nuked his own political career in the process. But if Garfield died, Conkling's lackey (Chester Arthur) would become president, and he would be the power behind the scenes and effectively run the country himself. He stood to benefit most of anyone from Garfield's assassination.
The assassin, Charles Guiteau, was obviously mentally ill. He had written an essay in praise of Garfield during the election and, like Conkling, felt responsible for Garfield's election and entitled to a job of his choosing (a consulship in Vienna or Paris, although he had no diplomatic qualifications and spoke no German or French). He was basically told to fuck off by James Blaine, Garfield's secretary of state. Considering the parallels between Guiteau and Conkling's situations, and the fact that they were aware of each other and had opportunity to meet during and after the campaign, I think it's possible that the idea to assassinate Garfield may have been planted in Guiteau's head by Conkling or one of his cronies. And perhaps Garfield's horrible and pushy doctor who lied his way into being Garfield's main physician allowed him to die a slow death of massive blood poisoning (he may have survived if treated by any physician who didn't stubbornly refuse to wash his hands/equipment) may have been sent over by Conkling as well. Or may have simply been a bad doctor and killed the president with medical malpractice (as Guiteau himself claimed). Why didn't Guiteau shoot Blaine instead of Garfield? Why did Guiteau have the mental clarity and empathy to be unable to shoot Garfield in the presence of Mrs Garfield, if he was truly as mentally unbalanced as he seemed?
It was probably just a random act of violence by a deranged man. But considering how desperate Conkling was and much he stood to gain, it does make me wonder.
Oddly enough, Chester Arthur had grown a pair by the time he was sworn in as president and his presidency gave a massive fuck-you to the Stalwart machine that had given him his power in the first place. Arthur served as a reformer, as Garfield would have wanted. He did, however, offer Conkling a position on the supreme court, which Conkling initially accepted. But apparently that wasn't enough power for old Roscoe, and he changed his mind and refused to serve.