The commercial tensions between England and the Netherlands were intensified when the English Parliament passed the Navigation Act 1651. This limited Dutch trade with any of the English colonies in America unless the shipping was done in "English bottoms" i.e. English ships. Indeed, any shipping coming into English ports or the ports of English colonies from anywhere in the world was required to be carried in English ships. Furthermore, the Navigation Act forbade all trade with those English colonies that retained connections and sympathy for the royalist cause of Charles I. To have accepted the terms of the Navigation Act was seen by the Dutch as agreeing to subordinate Dutch trade to the English trading system. This insulted Dutch pride and damaged their economy, but the more immediate cause of the war was the actions of the English navy and privateers against Dutch shipping. In 1651, 140 Dutch merchantmen were seized on the open seas. During January 1652 alone, another 30 Dutch ships were captured at sea and taken to English ports. Protests to England by the States General of the United Provinces were of no avail: the English Parliament showed no inclination toward curbing these seizures of Dutch shipping.