Between 1606 and 1608, De la Gardie served under the Dutch general Maurice of Nassau, Prince of Orange. Impressed with the Dutch way of waging war, De la Gardie began introducing Dutch methods into the Swedish army upon his return to the service of Sweden.
All but a few hundred of De la Gardie's men were killed or fled to the Polish side. This marked the failure of Charles IX of Sweden to place his son, Philip, on the Novgorodian throne. In 1617, De la Gardie became the chief Swedish negotiator at the Treaty of Stolbovo whereby Sweden was able to secure important territorial concessions from Russia, effectively closing off Russia from access to the Baltic Sea.
After 1621, De la Gardie took part in the Polish-Swedish War against his mother's half-brother King Sigismund III of Poland (former king of Sweden) in Livonia, but he was recalled after serving as commander in chief between 1626 and 1628. De la Gardie was an advocate of peace with Poland and acted as one of the Swedish negotiators at the Truce of Stuhmsdorf in 1635.
De la Gardie became a member of the state council of Sweden in 1613. In 1620 he became marshal and one of the five regents ruling Sweden during Queen Christina's minority (1632-44). His pacifist and pro-French and pro-Polish attitudes often put him at odds with chancellorAxel Oxenstierna, who led Sweden's war effort in the Thirty Years' War after the death of Gustavus Adolphus in 1632.
As De la Gardie supported many of Oxenstierna's other policies, eventually the two leaders reconciled after Oxenstierna's return to Sweden in 1636. Although the marshal's office came under criticism that year, De la Gardie continued to operate effectively, making large profits from leasing royal revenues and from loans to the crown.
Count Jacob De la Gardie died in Stockholm in 1652 and is buried in Veckholm church in Uppsala County. The city of Jakobstad in Finland is named after him.
Trivia
During the De la Gardie Campaign, the Finnish soldiers nick-named their commander Laiska-Jaakko ("Lazy Jacob"). This name is still widely remembered in Finland.