The United Provinces of the Netherlands, as the foremost Protestant state in continental Europe, was most obvious potential ally of the English Commonwealth. In 1651, two envoys from England, Oliver St. John and Walter Strickland, attempted to broker just such an alliance, but were refused. In response, since the two states were rivals in trade, the English Parliament passed the first Navigation Act, aimed at crippling the Dutch freight trade. It decreed that imports to England could only be carried on English ships, or those of their country of origin. It unsurprisingly created tension between the states, and contributed to the outbreak of the First Dutch War in 1652, over control of the sea and trade routes.