Long Parliament as Member of Parliament for Cardiff where he opposed compromising with the King, Charles I. In 1648 he opposed the purge of moderates who had formed the Rump Parliament. Despite being a commissioner for the trial of Charles, Sidney opposed the decision to execute the king, believing it to be of questionable lawfulness and wisdom. He said in explaining his view: By 1649 Sidney had changed his opinion, declaring the king's execution as "the justest and bravest act ... that ever was done in England, or anywhere". In 1653 when Cromwell's army entered Parliament to dissolve it after a Bill was introduced that would have made elections freer, Sidney refused to leave the House until threatened with physical removal. He regarded Cromwell as a tyrant. In retirement, Sidney was bold enough to outrage the Lord Protector by allegedly putting on a performance of Julius Caesar, and playing the role of Brutus. He was for a time the lover of Lucy Walter, later the mistress of Charles II. Sidney regarded the Republic as vigorously pursuing England's national interests (in contrast to the Stuarts' record of military failure), writing in his Discourses Concerning Government: Baltic ambassador After Cromwell's death in 1658, the army abolished the Protectorate in 1659 and reconvened the Rump Parliament, with Sidney taking up his seat in the Commons. During 1659–1660 he was part of a delegation to help arbitrate peace between Denmark and Sweden, as war would threaten England's naval supplies, as well as those of the Dutch. The delegation was commanded by Edward Montagu, with Sidney and Sir Robert Honywood. The third planned plenipotentiary, Bulstrode Whitelocke, declined because: "I knew well the overruling temper and height of Colonel Sydney". Sidney discarded conventional diplomatic norms ("a few shots of our cannon would have made this peace") to impose a peace favourable to England. Due to the Swedish king Charles X being unable to immediately receive them, the delegation negotiated with the Dutch on forming a joint fleet to impose peace terms. Charles X complained that the English "wish to command all, as if they were masters". Sidney in person handed Charles the treaty proposal (already accepted by Denmark), threatening military action. He recorded that Charles "in great choler ... told us, that we made projects upon our fleets, and he, laying his hand upon his sword, had a project by his side". Sidney would not back