Count Gustav Horn af Björneborg (October 22, 1592 – May 10, 1657) was a Swedish / Finnish soldier and politician. He was the youngest son of Field Marshal Carl Horn and Agneta von Dellwig, born while his father was imprisoned in Örbyhus Castle, after the defeat against the Russians.

He was born of the Finnish noble family Horn of Kankas, and was appointed member of the Royal Council in 1625, Field Marshal in 1628, Governor General of Livonia in 1652 and Lord High Constable since 1653. In the Thirty Years' War (1618 – 1648), he was instrumental as a commander in securing victory at the Battle of Breitenfeld, in 1631. He was High Councillor of the realm in 1625, elevated to rank of field marshal in 1628, and sometimes commander - in - chief of Swedish forces in Germany during Thirty Years' War. After the war, he served as Governor - General of Livonia 1652, President of War department and Lord High Constable in 1653. In 1651, Queen Christina created him Count of Pori.

He married Kristina Oxenstierna (1609 – 1631), daughter of Count Axel Oxenstierna, the Chancellor, in 1628, and fathered countess Agneta Horn (1629 – 1672), his heiress, who married baron Lars Cruus of Gudhem, Lord of Harviala. In 1643 he married his second wife Sigrid Bielke (born 1620), and fathered two more daughters: countess Eva Horn and countess Hedvig Lovisa Horn.

Young lord Gustav Horn studied extensively in European universities, and then took part in war against Russia. He studied military sciences under prince Maurice of Orange in the Netherlands.

As colonel, Gustav Horn took part in siege of Riga in 1621 and was seriously wounded. He led the troops which conquered Tartu in Livonian Estonia. With baron Johan De La Gardie, he led the defense of Livonia against Poland in late 1620s. Already at the age of 35, he was elevated to the rank of field marshal, by king Gustav II Adolf, also known as Gustavus Adolphus.

When the king decided to join the war in Germany (1630), he appointed Gustav Horn as his second in command. At the battle of Breitenfeld in 1631, Horn prevented the Imperial force, under Tilly, from flanking the main body of the Swedish army after their Saxon allies fled the field. Next Horn led troops in Upper (southern) Franconia and conquered, among others, Mergentheim, the town of the Teutonic Order, and the bishopric of Bamberg, then moved to Bavaria with the king. Horn was sent to lead troops in Rhineland, where he occupied Koblenz and Trier, and even continued to Swabia.

After the death of king Gustav II Adolf in the field at Lützen in November 1632, field marshal Horn and general John Banér were appointed to the overall command for Swedish forces in Germany. Gustav Horn's father - in - law, the Chancellor Oxenstierna, took the leadership of the civil government.

When Horn had to combine his troops with those of Bernhard of Weimar, all their co-operation failed, and they were given separate directions.

After Wallenstein's death in 1634, Horn took some areas in Swabia: in spring 1634, his troops were unsuccessful in capturing the imperial city of Überlingen, which would have been a rich and valuable prize. In late August/early September 1634, his forces, and those of Bernard of Saxony, were crushed at the battle of Nördlingen by combined Habsburg and Spanish forces. Horn was taken prisoner and held by the Catholic army in Burghausen Castle until 1642. He was exchanged for three imperial generals.

Following his exchange, Horn received his appointment as Vice President of the War department of the government. During the war against Denmark in 1644, Gustav Horn led the attack to Skåne and conquered the whole province, except the towns of Malmö and Kristianstad. Malmö's besieging did not yield results until the Treaty of Brömsebro stopped that war. Skåne's conquest became known as "the Horn War."

In 1651 he received Pori as his county and the castle of Marienborg, already earlier donated to him, was erected into a barony. Horn served then as Governor General in Livonia, and as Lord High Constable of the "empire", becoming thus Lord President of the War Department.

When the war against Poland was started in 1655, Gustav Horn directed the defense of Sweden, while offensive forces went to Poland.

Gustav Horn was one of the most capable of Gustav II Adolf's military commanders, and also an able administrator. His particular skills were in arranging defences for several sorts of situations. He also maintained relatively strict discipline, so his troops did not plunder and pillage as much as others.