With Christian IV of Denmark for many years the Swedes and the Dutch had had constant disputes and much friction.  This able and ambitious king, throughout a long and vigorous reign, which began in 1593, had watched with ever-increasing jealousy the passing of the Baltic trade into Dutch hands, and with something more than jealousy the rapid advance to power of the sister Scandinavian kingdom under Gustavus Adolphus.  Of the 1074 merchant ships that passed through the Sound between June 19 and November 16, 1645, all but 49 came from Dutch ports, by far the largest number from Amsterdam; and from these Christian IV drew a large revenue by the exaction of harsh and arbitrary toll-dues.  Again and again the States-General had complained and protested; and diplomatic pressure had been brought to bear upon the high-handed king, but without avail.  Between Sweden and Denmark there had been, since Gustavus Adolphus came to the throne in 1613, no overt act of hostility; but smouldering beneath the surface of an armed truce were embers of latent rivalries and ambitions ready at any moment to burst into flame.  Christian IV was a Protestant, but his jealousy of Sweden led him in 1639 openly to take sides with the Catholic powers, Austria and Spain.  Fearing that he might attempt to close the passage of the Sound, the States-General and the Swedish Regency in 1640 concluded a treaty “for securing the freedom and protection of shipping and commerce in the Baltic and North Seas”; and one of the secret articles gave permission to Sweden to buy or hire ships in the Netherlands and in case of necessity to enlist crews for the same.  Outward peace was precariously maintained between the Scandinavian powers, when the seizure of a number of Swedish ships in the Sound in 1643 made Oxenstierna resolve upon a bold stroke.  Without any declaration of war the Swedish general, Torstensson, was ordered to lead his victorious army from North Germany into Denmark and to force King Christian to cease intriguing with the enemy.  Holstein, Schleswig and Jutland were speedily in Torstensson’s hands, but the Danish fleet was superior to the Swedish, and he could make no further progress.  Both sides turned to the United Provinces.  Christian promised that the grievances in regard to the Sound dues should be removed if the States-General would remain neutral.  Oxenstierna addressed himself to Louis de Geer.  The merchant on behalf of the Swedish government was instructed to approach the stadholder and the States-General, and to seek for naval assistance under the terms of the treaty of 1640; and, if he failed in obtaining their assent, then he—­de Geer—­should himself (in conformance with the secret article of that treaty) raise on his own account and equip a fleet of thirty ships for the Swedish service.