He on his part in return for this was able to show in a remarkable way that he was not ungrateful for the favours that he had received.
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.