The king relied upon de Geer for the supply of all the necessaries for his armies in the field, and even commissioned him to raise troops for the Swedish service.

In 1626 the Dutch merchant was appointed by the king acting-manager of the copper mines, which were royal property; and, in order to regularise his position and give him greater facilities for the conduct of his enterprises, the rights of Swedish citizenship were conferred by royal patent upon him.

It was a curious position, for though de Geer paid many visits to Sweden, once for three consecutive years, 1626-29, he continued to make Amsterdam his home and principal residence.

He thus had a dual nationality.

Year after year saw an increasing number of mines and properties passing into the great financier's hands, and in return for these concessions he made large advances to the king for his triumphant expedition into Germany; advancing him in 1628 50,000 rixdalers, and somewhat later a further sum of 32,000 rixdalers.