The Good Old Cause was the name given, retrospectively, by the soldiers of the New Model Army, to the complex of reasons that motivated their fight on behalf of the Parliament of England.
Their struggle was against King Charles I and the Royalists during the English Civil War; they continued to support the English Commonwealth between 1649 and 1660. Oliver Cromwell spoke, in a letter to Sir William Spring in 1643, of the archetypal plain, russet-coated captain that knows what he fights for, and loves what he knows as the ideal of republican soldiery. (Many of those who supported the Good Old Cause were also Independents who advocated local congregational control of religious and church matters.)
Those who disagreed with expedient political compromises made during the period of the Commonwealth, went back to the Army's own declarations during the wars, to republican pamphlets like those produced by John Lilburne, Marchamont Nedham and John Milton. With their discontent, they imagined that there had been a moment of revolutionary purity when all these writers had agreed on something intrinsically republican and good — this entity, shifting as the sands depending upon the writer, was often labelled the "Good Old Cause".
After the death of Oliver Cromwell the phrase came into use gradually, passing to and fro in documents and speeches. By April 1659 and for months afterwards it was frequently heard on general discourse and every second or third pamphlet in the booksellers' shops had "The Good Old Cause" on its title-page or running through its text.
The phrase was open to interpretation, but in 1659 to its exponents it meant the pure Republican constitution which had been founded on the Regicide and which lasted until Cromwell's dissolution of the Rump Parliament on 20 April 1653. It proclaimed that Cromwell's interim dictatorship and Protectorate had been an interruption of the natural course of things, dexterously leaving it an open question whether that interruption had been necessary or justifiable, but calling on all men, now that Cromwell was dead and his effectiveness gone with him, to regard his rule as exceptional and extraordinary, and to revert to the old Commonwealth.