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.

His parents were the government councillor of Pomerania, Gerdt Antoniison Rehnskiöld (1610−1658), originally Keffenbrinck, and Birgitta Torskeskål (died 1655), niece of Baron Johan Adler Salvius.

Charles XI was highly impressed by Rehnskiöld's bravery, promoting him on the battlefield to major and transferring him to be Adjutant-General in the General Staff under Erik Dahlbergh's guidance and supervision.

After von Ascheberg's death in April 1693, Rehnskiöld came to finish his work of renewing the old allotment system, becoming Charles XI's chief military steward, and was employed in matters of tactics and education.

The King of Poland and Elector of Saxony, Augustus II, crossed the Düna river with his Saxon troops and besieged the city of Riga in Swedish Livonia.

Shortly before, Charles XII was informed that Russian troops under Tsar Peter I had besieged the strategically important Swedish outpost of Narva in Estonia.

[29][30] Together with Quartermaster Lieutenant-General Gerdt Ehrenschantz and artillery commander Johan Siöblad, Rehnskiöld drafted a simple battle plan that was never written on paper.

I have so much more reason to venerate him, who through his instigation that day I came to command on the most difficult side as Major-General, of which I have the pleasure to call myself.The main Swedish army overwintered outside the town of Dorpat and the dilapidated Laiuse Castle.

However, because of strong currents, the floating bridge was destroyed and its repair was prolonged, forcing Rehnskiöld to improvise by transporting parts of his own Life Dragoon Regiment with rafts.

He was tasked with securing the negotiations between the various noble factions in Warsaw, and to collect money and food from the immediate vicinity to supply the Swedish troops besieging the city of Thorn.

He established headquarters near Piotrków Trybunalski, while his troops were mostly stationed in different locations in Greater Poland, where he kept a watchful eye towards the nobility factions fighting for Augustus II.

In April 1703 Rehnskiöld was promoted to General of cavalry, elevating his prestige during his negotiations with the Polish nobility factions which included Prince James Louis Sobieski, magnate Hieronim Augustyn Lubomirski and Cardinal Michał Stefan Radziejowski.

The Saxon forces comprised a total of 25,000 men, reinforced by Russian auxiliaries and commanded by Field Marshal Johann Matthias von der Schulenburg.

Without any mercy in the circle, they were immediately shot and stabbed to death, they fell on top of each other like slaughtered sheep.His order was condemned by Swedish historians such as Eirik Hornborg, Sverker Oredsson, and Peter Englund.

[61][62][63] Other historians such as August Quennerstedt and Gustaf Adlerfelt considered that the massacre did not take place on Rehnskiöld's orders, but rather, might have occurred during the desperate situation in the final stages of the battle.

Under Swedish terms, Augustus II was forced to break all ties with his allies, renounce his claims to the Polish crown, and accept Stanisław Leszczyński as the new King.

Rehnskiöld and the army command were unaware of Charles XII's plans for the campaign, which the King kept to himself, but agreed upon a preliminary march towards Russia's capital Moscow, where Peter I had gathered most of his forces.

Later in the year, during the crossing of the Vabitj River at the town of Holowczyn in July 1708, the Swedish vanguard encountered a Russian army in fortified positions on the opposite shore.

[70][71] Following the battle of Holowczyn, Charles remained for nine weeks in Mogilev and the nearby areas to the east, between the Dnieper and its tributary Sozh, awaiting Lewenhaupt's late arrival.

[72][73] Throughout the campaign, Rehnskiöld held a fierce rivalry with the Marshal of the Realm, Count Carl Piper,[c] who had accompanied Charles XII as chief of the perambulating chancellery since 1700.

The antagonism between the two, in combination with their fiery temperament and pride in their own abilities, made them unable to reason with each other without an intermediary, a role usually filled by Quartermaster General Axel Gyllenkrok.

[90][88][91] A couple of days after the battle, Lewenhaupt and the 20,000 soldiers and non-combatants remaining from the Carolean Army, surrendered to Prince Menshikov at the village of Perevolochna at the ford of the Dnieper.

The King, Mazepa and about 1,000 men managed to cross the river and headed to the Ottoman Empire, where Charles stayed for several years before returning to Sweden at the end of 1715.

The first was that he organized no reconnaissance of the Russian redoubts that were built the night before the battle and had not informed his subordinates about his plan of attack, causing great confusion in the Swedish high command.

Rehnskiöld came to be involved in the matter of succession in Sweden on the initiative of Georg Heinrich von Görtz from Holstein-Gottorp, who had served as Swedish "grand-vizier" since 1716 and was responsible for the peace talks.

After pressure from the Swedish side, Peter I gave the order on 17 September to send Rehnskiöld to Lövö, where he would be exchanged with Major-General Ivan Trubetskoy and Count Avtonom Golovin.

Rehnskiöld arrived at Lovö on 14 October, where he entrusted Görtz's secretary, Andreas Stambke, to convince Charles Frederick to marry a daughter to Peter I in order to bolster relations between Sweden and Russia.

Furthermore, Frederick ordered the arrest of Görtz on 2 December, since the Hessian party sought to seize the upper hand in the matter of succession, which they gained with the death of Charles XII.

The reason for this was that a considerable sum of 100,000 riksdaler arrived from the war commissioner, and Prince Frederick wanted to distribute the money to the high command of the army, in order to win influential votes in the matter of succession.

Rehnskiöld later learned about Commandant Henrich Danckwardt's surrender of Carlsten Fortress and decided to stay in Uddevalla despite the threat of being cut off by Danish forces, who could be landed north of Gothenburg.