Before the sittings of the Great Assembly had come to an end, a young statesman, destined to play the leading part in the government of the Dutch republic during two decades, had already made his mark. After the death of William II Jacob de Witt was not only reinstated in his former position at Dordrecht but on December 21, 1650, John, his younger son, at the age of 25 years was appointed pensionary of that town. In this capacity he was ex officio spokesman of the deputation sent to represent Dordrecht in the Great Assembly. His knowledge, his readiness and persuasiveness of speech, his industry and his gifts at once of swift insight and orderly thoroughness, quickly secured for him a foremost place both in the deliberations of the Assembly and in the conduct of the negotiations with the English Parliament, which at this time required very delicate handling.

The many disputes, which had arisen between England and the United Provinces during the period between the accession of James I and the battle of the Downs in 1639, had never been settled. The minds of Englishmen were occupied with other and more pressing matters while the Civil War lasted. But the old sores remained open. Moreover the refusal of the States-General to receive the Parliamentary envoys, the murder of Doreslaer, and the protection afforded to royalist refugees, had been additional causes of resentment; but the English Council had not felt strong enough to take action. The death of the Prince of Orange, following so quickly upon the complete overthrow of Charles II at Worcester, appeared at first to open out a prospect of friendlier relations between the two neighbouring republics. In January, 1651, the Great Assembly formally recognised the Commonwealth and determined to send back to his old post in London the veteran ambassador, Joachimi, who had been recalled. The English government on their part anticipated his return by despatching, in March, Oliver St John and Walter Strickland on a special embassy to the Hague. They reached that city on March 27, 1651, and presented their credentials to the Great Assembly two days later. Their reception in the streets was anything but favourable. The feeling among the populace was predominantly Orangist and Stewart; and St John and Strickland, greeted with loud cries of "regicides" and many abusive epithets, remembering the fate of Doreslaer, were in fear of their lives.

On April 4 a conference was opened between the envoys and six commissioners appointed by the States to consider the proposals of the English Government for "a more strict and intimate alliance and union" between the two states. The Dutch quickly perceived that what the English really wanted was nothing less than such a binding alliance or rather coalition as would practically merge the lesser state in the greater. But the very idea of such a loss of the independence that they had only just won was to the Netherlanders unthinkable. The negotiations came to a deadlock. Meanwhile St John and Strickland continued to have insults hurled at them by Orangists and royalist refugees, foremost amongst them Prince Edward, son of the Queen of Bohemia. The Parliament threatened to recall the envoys, but consented that they should remain, on the undertaking of the Estates of Holland to protect them from further attacks, and to punish the offenders. New proposals were accordingly made for an offensive and defensive alliance (without any suggestion of a union), coupled with the condition that both States should bind themselves not to allow the presence within their boundaries of avowed enemies of the other - in other words the expulsion of the members and adherents of the house of Stewart, including the princess royal and the Queen of Bohemia with their children. In the face of the strong popular affection for the infant Prince of Orange and his mother, even the Estates of Holland dared not consider such terms, and the States-General would have angrily rejected them. After some further parleying therefore about fisheries and trade restrictions, it was felt that no agreement could be reached; and St John and Strickland returned to England on July 31, 1651.

Their failure created a very bad impression upon the Parliament. All the old complaints against the Dutch were revived; and, as they had refused the offer of friendship that had been made to them, it was resolved that strong measures should be taken to obtain redress for past grievances and for the protection of English trade interests.