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." This expedition under Marten Thijssen, who after his victory was created a Swedish noble and definitely entered the Swedish naval service, though connived at by Frederick Henry and the States-General, did not express any desire on their part to aggrandise Sweden unduly at the expense of Denmark.

If some great merchants such as Louis de Geer and Elias Trip were exploiting the resources of Sweden, others, notably a certain Gabriel Marcelis, had invested their capital in developing the Danish grazing lands; and politically and commercially the question of the Sound dues, pre-eminently a Danish question, overshadowed all others in importance.

The Dutch had no desire to give Sweden a share in the control of the Sound; they preferred in the interests of their vast Baltic trade to have to deal with Christian IV alone.

The Swedish threat was useful in bringing diplomatic pressure to bear on the Danish king, but ultimately they felt confident that, if he refused to make concessions in the matter of the dues, they could compel him to do so.
As one of their diplomatists proudly declared, "the wooden keys of the Sound were not in the hands of King Christian, but in the wharves of Amsterdam." In June, 1645, his words were put to a practical test.
Admiral Witte de With at the head of a fleet of fifty war-ships was ordered to convoy 300 merchantmen through the Sound, peacefully if possible, if not, by force.