The tale that the countess, disguised as a page, witnessed the encounter, appears to have no foundation; but Buckingham, by installing the "widow of his own creation" in his own and his wife's house, outraged even the lax opinion of that day. He was thought to have originated the project of obtaining the divorce of the childless queen. He intrigued against James, against Sir William Coventry - one of the ablest statesmen of the time, whose fall he procured by provoking him to send him a challenge - and against the great duke of Ormonde, who was dismissed in 1669. He was even suspected of having instigated Thomas Blood's attempt to kidnap and murder Ormonde, and was charged with the crime in the king's presence by Ormonde's son, Lord Ossory, who threatened to shoot him dead in the event of his father's meeting with a violent end. Arlington, next to Buckingham himself the most powerful member of the cabal and a favourite of the king, was a rival less easy to overcome; and he derived considerable influence from the control of foreign affairs entrusted to him.
Buckingham had from the first been an adherent of the French alliance, while Arlington concluded through Sir William Temple in 1668 the Triple Alliance. But on the complete volte-face and surrender made by Charles to France in 1670, Arlington as a Roman Catholic was entrusted with the first treaty of Dover of the 20th of May - which besides providing for the united attack on Holland, included Charles's undertaking to proclaim himself a Romanist and to reintroduce the Roman Catholic faith into England, - While Buckingham was sent to France to carry on the sham negotiations which led to the public treaties of the 31st of December 1670 and the 2nd of February 1672. He was much pleased with his reception by Louis XIV., declared that he had "more honours done him than ever were given to any subject," and was presented with a pension of 10,000 livres a year for Lady Shrewsbury. In June 1672 he accompanied Arlington to the Hague to impose terms on the prince of Orange, and with Arlington arranged the new treaty with Louis. After all this activity he suffered a keen disappointment in being passed over for the command of the English forces in favour of Schomberg. He now knew of the secret treaty of Dover, and towards the end of 1673 his jealousy of Arlington became open hostility.
He threatened to impeach him, and endeavoured with the help of Louis to stir up a faction against him in parliament. This, however, was unsuccessful, and in January 1674 an attack was made upon Buckingham himself simultaneously in both houses. In the Lords the trustees of the young earl of Shrewsbury complained that Buckingham continued publicly his intimacy with the countess, and that a son of theirs had been buried in Westminster Abbey with the title of earl of Coventry; and Buckingham, after presenting an apology, was required, as was the countess, to give security for £10,000 not to cohabit together again. In the Commons he was attacked as the promoter of the French alliance, of "popery" and arbitrary government. He defended himself chiefly by endeavouring to throw the blame upon Arlington; but an address was voted petitioning the king to remove him from his councils, presence and from employment for ever. Charles, who had only been waiting for a favourable opportunity, and who was enraged at Buckingham's disclosures, consented with alacrity. Buckingham retired into private life, reformed his ways, attended church with his wife, began to pay his debts, became a "patriot," and was claimed by the country or opposition party as one of their leaders.
In the spring of 1675 he was conspicuous for his opposition to the Test oath and for his abuse of the bishops, and on the 16th of November he introduced a bill for the relief of the nonconformists. On the 15th of February 1677 he was one of the four lords who endeavoured to embarrass the government by raising the question whether the parliament, not having assembled according to the act of Edward III. once in the year, had not been dissolved by the recent prorogation. The motion was rejected and the four lords were ordered to apologize. On their refusing, they were sent to the Tower, Buckingham in particular exasperating the House by ridiculing its censure. He was released in July, and immediately entered into intrigues with Barillon, the French ambassador, with the object of hindering the grant of supplies to the king; and in 1678 he visited Paris to get the assistance of Louis XIV. for the cause of the opposition. He took an active part in the prosecution of those implicated in the supposed Popish Plot, and accused the lord chief justice (Sir William Scroggs) in his own court while on circuit of favouring the Roman Catholics. In consequence of his conduct a writ was issued for his apprehension, but it was never served.
He promoted the return of Whig candidates to parliament, constituted himself the champion of the dissenters, and was admitted a freeman of the city of London. He, however, separated himself from the Whigs on the exclusion question, probably on account of his dislike of Monmouth and Shaftesbury, was absent from the great debate in the Lords on the 15th of November 1680, and was restored to the king's favour in 1684.
He took no part in public life after James's accession, but returned to his manor of Helmsley in Yorkshire, the cause of his withdrawal being probably exhausted health and exhausted finances. In 1685 he published a pamphlet, entitled A short Discourse on the Reasonableness of Man's having a Religion (reprinted in Somers Tracts (1813, ix. 13), in which after discussing the main subject he returned to his favourite topic, religious toleration. The tract provoked some rejoinders and was defended, amongst others, by William Penn, and by the author himself in The Duke of Buckingham's Letter to the unknown author of a short answer to the Duke of Buckingham's Paper (1685). In hopes of converting him to Roman Catholicism James sent him a priest, but Buckingham turned his arguments into ridicule. He died on the 16th of April 1687, from a chill caught while hunting, in the house of a tenant at Kirkby Moorside in Yorkshire, expressing great repentance and feeling himself "despised by my country and I fear forsaken by my God." The miserable picture of his end drawn by Pope, however, is greatly exaggerated. He was buried on the 7th of June 1687 in Henry VII.'s chapel in Westminster Abbey, in greater state, it was said, than the late king, and with greater splendour.