In deep secrecy De Witt now made preparations for the despatch of a great fleet with orders to sail up the estuary of the Thames and attack the English ships in harbour.  De Ruyter, accompanied by Cornelis de Witt, left the Texel on June 14, at the head of a fleet numbering more than eighty vessels.  A squadron under Admiral Van Ghent sailed up the Thames on June 19, followed by the main body.  Sheerness was captured, and on the 22nd De Ruyter determined to force his way up the Medway.  The river had been blocked by drawing up a line of ships behind a heavy chain.  The Dutch fire-ships broke through the chain and burnt the vessels, and then proceeding upwards burnt, scuttled or captured some sixteen vessels, among the latter the flag-ship, Royal Charles.  The sound of the Dutch guns was heard in London and for a time panic reigned.  But the narrowness of the river and the prompt measures that were taken to call out the militia and man the forts prevented any further success.  The Dutch fleet withdrew to the Nore and, beyond blocking the mouth of the river, were able to effect no further damage.  The blow to English prestige was however irreparable, and the people felt deeply humiliated that short-sightedness and lack of preparation on the part of the government should have exposed them to an insult galling to the national pride.  One of its consequences, as had been anticipated by De Witt, was a more conciliatory attitude on the part of the English envoys at Breda.  Peace was concluded on July 26, on terms more favourable than the Dutch could have expected.  The Navigation Act was modified, various commercial advantages were conceded and Poeloe-Rum was retained.  On the other hand, the custom of the striking of the flag remained unchanged.  It was agreed that the English colony of Surinam, which had been captured in March, 1667, by a Zeeland squadron should be kept in exchange for New York, an exchange advantageous to both parties.

By the treaty of Breda the Dutch republic attained the summit of its greatness, and the supremacy of De Witt appeared to be not only secure but unassailable.  Yet events were preparing which were destined to undermine the prosperity of Holland and the position of the statesman to whom in so large a measure that prosperity was due.  France under the absolute rule of Louis XIV had become by far the most powerful State in Europe, and the king was bent upon ambitious and aggressive projects.  It has already been explained that after the death of Philip IV of Spain he claimed for his queen, Maria Theresa, the succession, by the so-called “law of devolution,” to a large part of the southern Netherlands.  He now determined that the hour had come for enforcing his claim.  In May, 1667, before the treaty of Breda had been signed, a French army of 50,000 men crossed the Belgic frontier.  Castel-Rodrigo, the Spanish governor, had no force at his disposal for resisting so formidable an invasion; fortress after fortress fell into French hands; and