De Re Publica, variously translated as The Republic, On the Republic or On the Commonwealth, is Cicero's major work on political theory. It was written between 54 and 51 BCE, years when Cicero was politically marginalised by the First Triumvirate whose break-up would shortly lead to civil war.
The dialogue is set in an earlier turbulent period in the preceding century. It's central character is Scipio Aemilianus, the victorious general of the Third Punic War, and leader of the aristocratic opposition to the popular faction of the Gracchi.
The Middle Ages knew only a single major fragment of the De Re Publica, the passage known as Scipio's Dream. A palimpsest found in the Nineteenth Century contributed to the other surviving portions.
Free online texts
Internet Archive: The treatises of M.T. Cicero: On the nature of the gods; On divination; On fate; On the republic; On the laws; and On standing for the consulship. Literally translated chiefly by the editor, C.D. Yonge (1878). Multiple formats.
Latin Library: De Re Republica. Latin text, HTML format.
Online Library of Liberty: The Political Works of Marcus Tullius Cicero, vol. 1 (Treatise on the Commonwealth), translated by Francis Barham (1841). Multiple formats.
Perseus: De Republica (Teubner, 1889). Latin text, HTML and XML format.
Wikisource: De Re Publica. Latin text, multiple formats.
History of Philosophy without any gaps: Rhetorical Questions: Cicero - podcast by philosopher Peter Adamson.
History of Philosophy without any gaps: Raphael Woolf on Cicero - podcast by philosopher Peter Adamson.
Internet Encyclopedia of Philosophy: Cicero.
Stanford Encyclopedia of Philosophy: Ancient Political Philosophy.
The Great Conversation: Further reading at Tom's Learning Notes
Cicero: Letters to his Brother Quintus - include discussion of the composition of the work
Plato: The Republic.
Polybius: The Histories - significant precursor of Cicero's ideas on the mixed constitution.
Latin resources: Learn to read Latin texts in the original.