Short answers, pulled from the story.
Treason is the crime of attacking a state authority to which one owes allegiance. Common acts include participating in a war against one's native country, attempting to overthrow its government, spying for a hostile foreign power, or attempting to kill its head of state. A person who commits treason is known in law as a traitor.
The words treason and traitor derive from the Latin tradere, meaning "to deliver or hand over." Specifically, they trace to traditors, the term for bishops and Christians who handed sacred scriptures or fellow believers to Roman authorities during the Diocletianic Persecution between AD 303 and 305.
The Treason Act 1351 was the first time treason was defined by statute in English law. Legal scholar Joseph Story called it "the pole star of English jurisprudence." It established categories of treasonable offences including plotting the king's death, levying war against him, and violating his closest female relatives, and it remains part of British law today.
William Joyce, known as Lord Haw-Haw, was the last person executed for treason in Britain, hanged in 1946. Joyce had broadcast Nazi propaganda from Germany to the United Kingdom during the Second World War. He was convicted because the court ruled he owed allegiance to the British Crown after having used a British passport.
The Expatriation Act of 1868 was passed by the United States Congress to grant Americans the right to freely renounce their citizenship. It arose directly from the controversy after the 1867 Fenian Rising, when Britain charged Irish-Americans who had joined the uprising with treason under the doctrine of perpetual allegiance, which held that British subjects remained subjects even after emigrating and taking foreign citizenship.
Marshal Michel Ney swore allegiance to King Louis XVIII of France after Napoleon's first defeat, then rejoined Napoleon and commanded French forces at the Battle of Waterloo. After Napoleon's final exile in the summer of 1815, Ney was arrested and tried by the Chamber of Peers. His lawyer argued he had become a Prussian citizen under the Treaty of Paris and thus owed no loyalty to France, but Ney rejected this defence by declaring publicly that he was French and would remain French, sealing his conviction and execution.