The Treaty of Paris signed in March 1657 allied the English Protectorate of Oliver Cromwell with King Louis XIV of France against King Philip IV of Spain, merging the Anglo-Spanish War (1654–1660) with the larger Franco-Spanish War (1635–1659).
The treaty confirmed the growing rapprochement between France and the English Republican regime.
England would contribute both 6,000 troops and its fleet in a campaign against the Flemish coastal fortresses of Gravelines, Dunkirk and Mardyck.
Dunkirk, in particular, was on the Commonwealth's mind mainly because of the privateers that were causing damage to the mercantile fleet.
For Cromwell and the Commonwealth, the question of possession of Dunkirk thus passed from a diplomatic possibility in the region to urgent political necessity.