The Treaty of Westminster, concluded between the Lord Protector of the English Commonwealth, Oliver Cromwell, and the States General of the United Netherlands, was signed on 5/15 April 1654.

A secret clause, obliging the States of Holland to enact the Act of Seclusion, played an important part in Dutch internal politics during the First Stadtholderless Period.

Older established states, like the Dutch Republic, looked somewhat askance at the "upstart" England, which was ruled by "king killers."

The death of William II in 1650 and the establishment of a new States Party regime in the Dutch Republic cleared the way for a thaw in Anglo-Dutch diplomatic relations.

The Dutch government replied with a number of counter-proposals in matters they held dear, like principles of (then-emerging) international law, such as freedom of the seas, limitation of the concept of contraband to "instruments of war", freedom from capture of neutral goods on neutral ships in time of war.

[1]: 7–8 On 9/19 October 1651 the Commonwealth Parliament enacted the first of the Navigation Acts, which was perceived as aimed at the ruination of Dutch commerce,[c] possibly on the instigation of St. John, who was aggrieved by his cool treatment in The Hague.

Remarkably, the English offered at this time to acquiesce in this exclusion from the East Indies, but insisted on retaining the restrictions in the Navigation Act.

[6]: 721–722 Lord Protector Oliver Cromwell and Grand Pensionary Johan de Witt agreed to open peace negotiations in London in June 1653.

"[1]: 10–11 After Jongestal and Nieupoort had returned to The Hague to receive instructions regarding this startling proposal in mid-August 1653, the English side toned down the proposal in favor of a less far-reaching proposal for an alliance between the Protestant Powers of Europe and France, against the Catholic Powers, in which the Commonwealth and the Republic would take the lead; a commission, consisting of an equal number of members from each country, to determine differences peacefully; a joint fleet to "secure the sea;" liberty of the inhabitants of both countries to trade in Europe and overseas; the assignment of trade in East Asia solely to the Dutch in exchange for indemnification of the E.I.C.

[1]: 11 Jongestal and Nieuport returned in November 1653 with instructions to reject the proposed union, but to seek a close alliance commensurate with maintenance of Dutch independence.

Secondly, Cromwell demanded that the then-four-year-old Prince of Orange[e] should be excluded from future government appointments, like the stadtholderate, or the captaincy-general of the States Army.

Secret direct negotiations between Cromwell and De Witt (represented by Beverningh), led to agreement on the matter of the exclusion of the Prince, however without the knowledge of the States General of the Netherlands.

[6]: 722  The Dutch ambassadors returned to England after a few weeks' absence (only the representatives of Holland being aware of the secret clause) and completed the negotiations, tying up a number of loose ends, especially the important provisions in art.

The Act of Seclusion remained in effect, playing a part in Dutch internal politics, until the very end of the De Witt regime, even after the Restoration seemingly obviated its existence.

[6]: 722–726 The British historian Jonathan Israel writes that otherwise, taking into account that the Republic lost the war militarily, the treaty was remarkably lenient for the Dutch.

He ascribes this to the diplomatic acumen of De Witt, who managed to avoid doing any major concession to England's maritime and colonial interests.

New here was that the principle of binding arbitration by a third party (the Protestant Swiss Cantons[i]) was first agreed on as a means of conflict resolution in an international treaty.

This first of all targeted the pretender Charles Stuart, who had since 1648 found asylum in the Republic at the court of his sister Mary and her husband stadtholder William II.

Ironically, a similar clause was to be included in the treaties Charles II himself concluded with the Republic in 1662, 1667 and 1674, after his Restoration, but then aimed at the regicides.