On his return he led political negotiations with the Elector Augustus of Saxony and Frederick I of Prussia about the impending plans of war against Sweden.
For much of Frederick IV's reign Denmark-Norway was engaged in the Great Northern War (1700–1721) against Sweden.
In spite of the conclusion of the Peace of Travendal in 1700, there was soon a Swedish invasion and threats from Europe's western naval powers.
In 1709 Denmark again entered the war encouraged by the Swedish defeat at Poltava.
Frederick IV commanded the Danish troops at the Battle of Gadebusch in 1712.
Although Denmark-Norway emerged on the victorious side, she failed to reconquer lost possessions in southern Sweden.
The most important result was the destruction of the pro-Swedish Duchy of Holstein-Gottorp, which re-established Denmark's domination in Schleswig-Holstein.
Between 1703 and 1711, Frederick send military units in Hungary and supported Austria in the Rákóczi's War of Independence.
The Danish regiments fought against the Kuruc army and French auxiliaries (Battle of Zsibó).

Much of the king's life was spent in strife with kinsmen.
Two of his first cousins, Charles XII of Sweden and Frederick IV, Duke of Holstein-Gottorp (the three men were the grandsons of Frederick III of Denmark), had waged war upon his father jointly.
Initially defeated by the Swedes and forced to recognize the independence of Holstein-Gottorp, Frederick finally drove the next duke of Holstein-Gottorp, Duke Charles Frederick (who was Frederick IV's first cousin once removed) out of Schleswig in 1713, and avoided the revenge contemplated by Charles Frederick's mother-in-law, Catherine I of Russia.
On his return he led political negotiations with the Elector Augustus of Saxony and Frederick I of Prussia about the impending plans of war against Sweden. For much of Frederick IV's reign Denmark-Norway was engaged in the Great Northern War (1700–1721) against Sweden. In spite of the conclusion of the Peace of Travendal in 1700, there was soon a Swedish invasion and threats from Europe's western naval powers. In 1709 Denmark again entered the war encouraged by the Swedish defeat at Poltava. Frederick IV commanded the Danish troops at the Battle of Gadebusch in 1712. Although Denmark-Norway emerged on the victorious side, she failed to reconquer lost possessions in southern Sweden. The most important result was the destruction of the pro-Swedish Duchy of Holstein-Gottorp, which re-established Denmark's domination in Schleswig-Holstein. Between 1703 and 1711, Frederick send military units in Hungary and supported Austria in the Rákóczi's War of Independence. The Danish regiments fought against the Kuruc army and French auxiliaries (Battle of Zsibó).
Much of the king's life was spent in strife with kinsmen. Two of his first cousins, Charles XII of Sweden and Frederick IV, Duke of Holstein-Gottorp (the three men were the grandsons of Frederick III of Denmark), had waged war upon his father jointly. Initially defeated by the Swedes and forced to recognize the independence of Holstein-Gottorp, Frederick finally drove the next duke of Holstein-Gottorp, Duke Charles Frederick (who was Frederick IV's first cousin once removed) out of Schleswig in 1713, and avoided the revenge contemplated by Charles Frederick's mother-in-law, Catherine I of Russia.
Frederick IV of Denmark