Sidney was later charged with plotting against Charles II, in part based on his most famous work, Discourses Concerning Government, which was used by the prosecution as a witness at his trial.

[1] Despite having earlier vowed that only "extreme necessity shall make me think of bearing arms in England", Sidney served in the Army of the Eastern Association, becoming lieutenant colonel of the Earl of Manchester's regiment of horse (cavalry).

[4] He fought at the Battle of Marston Moor in 1644, where an observer wrote: "Colonel Sidney charged with much gallantry in the head of my Lord Manchester's regiment of horses, and came off with many wounds, the true badges of his honor".

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.

This being alleged in vain, and Cromwell using these formal words (I tell you, wee will cut off his head with the crown upon it) I ... immediately went out of the room, and never returned.

[1] 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.

[7] 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.

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: ... such was the power and wisdom and integrity in those that sat at the helm, and their diligence in causing men only for their merit was blessed with such success, that in two years our fleets grew to be as famous as our land armies; the reputation and power of our nation rose to a greater height, than when we possessed the better half of France, and the kings of France and Scotland were our prisoners.

[8] 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 and 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.

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.

[citation needed] Sidney in person handed Charles the treaty proposal (already accepted by Denmark), threatening military action.

In 1663 during a trip to the Calvinist academy at the University of Geneva, Sidney wrote in the visitor's book: "SIT SANGUINIS ULTOR JUSTORUM" ("Let there be revenge for the blood of the just").

It would naturally draw many of the Dutch to leave their country, that could not be kept and maintained but at a vast charge, and to exchange that with the plenty and security that England afforded.

Louis subsequently wrote that Sidney "promised me to produce a great uprising ... but the proposition he put to me to advance him 100,000 ecus ... was more than I wished to expose on the word of a fugitive [so] I offered him [initially] only 20,000".

Here he became involved in politics, with the French ambassador, Paul Barillon writing on 6 October: At the moment my most intimate liaison is with Mr. Algernon Sidney; he is the man in England who seems to me to have the greatest understanding of affairs; he has great relations with the rest of the Republican party; And nobody in my opinion is more capable of rendering service than him.

Lord Chief Justice Jeffreys (whose conduct of the trial caused much criticism, then and later) ruled: "Scribere est agere" ("to write is to act").

In The Apology of Algernon Sydney, in the Day of his Death, Sidney wrote that his life's work was to: ... uphold the Common rights of mankind, the lawes of this land, and the true Protestant religion, against corrupt principles, arbitrary power and Popery .

On the scaffold, Sidney argued that his conviction was unlawful, disputing the quality of the evidence against him and pointing out various deviations from proper legal procedure at his trial.

Still, he observed that he was abstaining from tackling truly "great matters" in this last speech of his, because "We live in an age that makes truth pass for treason".

His Discourses Concerning Government (the text for which Sidney lost his life) was written during the Exclusion Crisis, as a response to Robert Filmer's Patriarcha, a defence of divine right monarchy, first published in 1680.

This suggests a limited civil government whose primary purpose is to 1) render legal justice through its court system and 2) provide for the safety of its citizens.

He was stiff to all republican principles, and such an enemy to every thing that looked like monarchy, that he set himself in a high opposition against Cromwell when he was made protector.

[14] Sidney's reputation suffered a blow when Sir John Dalrymple published his Memoirs of Great Britain and Ireland in 1771, which exposed him as a pensioner of Louis XIV.

As splendid an edition of it as the art of printing can produce—as well for the intrinsic merit of the work, as for the proof it brings of the bitter sufferings of the advocates of liberty from that time to this, and to show the slow progress of moral, philosophical, and political illumination in the world—ought to be now published in America.

[18] The Whig historian Thomas Babington Macaulay said of Sidney in 1828: Never was there less of national feeling among the higher orders than during the reign of Charles the Second.

Algernon Sidney, on the other hand, would gladly have aided France in all her ambitious schemes, and have seen England reduced to the condition of a province, in the wild hope that a foreign despot would assist him to establish his darling republic.

[19] But in 1848, Macaulay wrote of the Whig opposition to Charles II: It would be unjust to impute to them the extreme wickedness of taking bribes to injure their country.

Among those who cannot be acquitted of this degrading charge was one man who is popularly considered as the personification of public spirit, and who, in spite of some great moral and intellectual faults, has a just claim to be called a hero, a philosopher, and a patriot.

Yet it is some consolation to reflect that, in our time, a public man would be thought lost to all sense of duty and of shame, who should not spurn from him a temptation which conquered the virtue and the pride of Algernon Sidney.