In the Great Northern War he was Charles XII's right-hand man in the operative organization of the Carolean Army and drafted the battle plans for the landing at Humlebæk and for the battles of Narva, Düna and Kliszów. In the battle of Fraustadt in 1706, with his own independent army, he decisively defeated a Saxon-Russian Army under the command of Field Marshal Schulenburg. For his services, Rehnskiöld was appointed Field Marshal and the title of Count. During Charles XII's campaign against Russia, Rehnskiöld was in command during the battle of Holowczyn and the siege of Veprik, where he was severely injured. After Charles XII became incapacitated by a bullet wound, Rehnskiöld replaced him as commander-in-chief of the Swedish Army during the battle of Poltava in 1709, where it suffered a decisive defeat.