The English Civil War climaxed with a trial unprecedented in British history. After years of conflict between the monarchy and Parliament, King Charles I was convicted of treason by a special court dominated by his opponents. On 30 January 1649, he was led onto a scaffold outside the Banqueting House in Whitehall and beheaded.

History.com notes that he was condemned for dissolving Parliament, attempting to rule alone and bringing war upon his own people. His execution abolished the monarchy for a decade and ushered in the short‑lived Commonwealth under Oliver Cromwell.

The French Revolution also produced one of the most famous regicides in history. In December 1792, Citizen Louis Capet (the deposed Louis XVI) was tried by the National Convention and found guilty of high treason. World History Encyclopedia records that the former king was sentenced to death by guillotine and executed in Paris on 21 January 1793. His death ended centuries of Bourbon rule and marked the radicalisation of the Revolution.

Russia’s last tsar was killed not on a public scaffold but in a basement in Yekaterinburg. After months under house arrest during the Bolshevik Revolution, Nicholas II and his family were murdered by Bolshevik guards on 17 July 1918. The execution was ordered by local Bolshevik authorities; ThoughtCo notes that Nicholas, his wife Alexandra and their children were shot and bayoneted, bringing an abrupt end to the Romanov dynasty.

In the 1860s, Napoleon III of France imposed Archduke Ferdinand Maximilian of Austria as emperor of Mexico. When French troops withdrew and republican forces rallied under Benito Juárez, the empire collapsed. A History.com account records that Maximilian was captured and, on 19 June 1867, executed by firing squad in Querétaro after France abandoned him. His death symbolised Mexico’s rejection of foreign-imposed monarchy and restored the republic.

The Hashemite monarchy in Iraq met a bloody end on the 14 July 1958 revolution. A group of nationalist army officers led by Abd al‑Karim Qasim stormed the royal palace. According to the article on Faisal II in Wikipedia, the young king “was killed during the 14 July Revolution” and “executed along with numerous members of his family” at the Al‑Rehab Palace.

The coup abolished the monarchy and established the Republic of Iraq, shifting the country’s alignment during the Cold War.