Early in the First English Civil War the Long Parliament threatened to retaliate in kind if the Royalists tried and executed John Lilburne and two other Parliamentary offices for treason.
Of five prominent Royalist peers who fell into the hands of Parliament, three, the Duke of Hamilton, the Earl of Holland, and Lord Capel, one of the Colchester prisoners and a man of high character, were beheaded at Westminster on 9 March.
Above all, after long hesitations, even after renewal of negotiations, the Grandees of the New Model Army and the Independents conducted "Pride's Purge" of the House removing their ill-wishers, and created the High court of Justice for the trial and sentence of King Charles I.
[5] At the end of the trial the 59 Commissioners (judges) found "Charles Stuart, that man of blood" guilty of high treason, as a "tyrant, traitor, murderer and public enemy".
The terms of the ordinance as the name suggests decree that no quarter should be given on the capture of any Irish Catholics found fighting for the Royalists in England or Wales (Scotland was another realm and under a different jurisdiction).