De Geer soon discovered that Frederick Henry, being intent on peace negotiations, was averse to the proposal.  The stadholder, and the States-General acting under his influence, did not wish to create fresh entanglements by embroiling the United Provinces in a war with Denmark.  De Geer therefore at once began on his own responsibility to equip ships in the various seaports of Holland and Zeeland which had been the chief sufferers by the vexatious Sound dues, and he succeeded in enlisting the connivance of the Estates of Holland to his undertaking.  Before the end of April, 1644, a fleet of thirty-two vessels was collected under the command of Marten Thijssen.  Its first efforts were unsuccessful.  The Danish fleet effectually prevented the junction of Thijssen with the Swedes, and for a time he found himself blockaded in a narrow passage called the Listerdiep.  Taking advantage of a storm which dispersed the Danes, the Dutch admiral at last was able to put to sea again, and early in July somewhat ignominiously returned to Amsterdam to refit.  For the moment King Christian was everywhere triumphant.  On July 11 he gained a signal victory over the Swedish fleet at Colberg Heath, and he had the satisfaction of seeing Torstensson compelled by the Imperialists to retreat from Jutland.  But the energy and pertinacity of the Amsterdam merchant saved the situation.  Though the retreat of Thijssen meant for him a heavy financial loss, de Geer never for a moment faltered in his purpose.  Within three weeks Thijssen again put to sea with twenty-two ships, and by skilful manoeuvring he succeeded in making his way through the Skagerak and the Sound, and finally brought his fleet to anchor in the Swedish harbour of Calmar.  From this harbour the united Swedo-Dutch squadrons sailed out and on October 23, between Femern and Laaland, met the Danish fleet, and after a desperate conflict completely defeated and destroyed it.  Thus were the wealth and resources of a private citizen of Amsterdam able to intervene decisively at a critical moment in the struggle for supremacy in the Baltic between the two Scandinavian powers.  But it is not in the victory won by Marten Thijssen that de Geer rendered his greatest service to Sweden.  As the Swedish historian Fryxell truly says, “all that was won by the statesmanship of Oxenstierna, by the sword of Baner, Torstensson and Wrangel, in a desolated Germany streaming with blood, has been already lost again; but the benefits which Louis de Geer brought to Sweden, by the path of peaceful industry and virtue, these still exist, and bear wholesome fruit to a late posterity.”