He was the eldest son of King Oscar II of Sweden and Sophia of Nassau, a half-sister of Adolphe, Grand Duke of Luxembourg.

During World War II, he allegedly urged Per Albin Hansson's coalition government to accept requests from Nazi Germany for logistics support, arguing that refusing might provoke an invasion.

Gustaf V was born on 16 June 1858 in Drottningholm Palace in Ekerö, Stockholm County, the son of Prince Oscar, Duke of Östergötland and Princess Sofia of Nassau.

At birth he was created Duke of Värmland, and on 12 July he was baptised Oscar Gustaf Adolf at the Royal Chapel of the Stockholm Palace by the Archbishop of Uppsala, Henrik Reuterdahl.

Among the prince's classmates at the school was Hjalmar Branting, who went on to become leader of the Swedish Social Democratic Party and three times Prime Minister of Sweden.

Early in his reign, in 1910, Gustaf V refused to grant clemency to the convicted murderer Johan Alfred Ander, who thus became the last person to be executed in Sweden.

In his reply, the so-called Courtyard Speech—which was actually written by explorer Sven Hedin, an ardent conservative—Gustaf promised to strengthen the country's defences.

The Edén government promptly arrogated most of the king's political powers to itself and enacted numerous reforms, most notably the institution of complete (male and female) universal suffrage in 1918–1919.

Parliamentarianism had become a de facto reality in Sweden, even if it would not be formalised until 1974, when a new Instrument of Government stripped the monarchy of even nominal political power.

In 1938, for instance, he personally summoned the German ambassador to Sweden and told him that if Hitler attacked Czechoslovakia over its refusal to give up the Sudetenland, it would trigger a world war that Germany would almost certainly lose.

Both the King and his grandson Prince Gustaf Adolf socialised with Nazi leaders before World War II, though arguably for diplomatic purposes.

During a visit to Berlin, according to historian Jörgen Weibull, Gustaf V attempted to convince Hitler to soften his persecution of the Jews.

When Nazi Germany invaded the Soviet Union in June 1941, Gustaf V tried to write a private letter to Hitler thanking him for taking care of the "Bolshevik[6] pest" and congratulating him on his "already achieved victories".

[9][10][11] According to Prime Minister Hansson, during the Midsummer crisis, the King in a private conversation had threatened to abdicate if the government did not approve a German request to transfer a German infantry division, the so-called Engelbrecht Division, through Swedish territory from southern Norway to northern Finland in June 1941, around Midsummer.

The accuracy of the claim is debated, and the King's intention, if he really made the threat, is sometimes alleged to be his desire to avoid conflict with Germany.

On a visit to Berlin, Gustaf went straight from a meeting with Hitler to a tennis match with the Jewish player Daniel Prenn.

During World War II, he interceded to obtain better treatment for Davis Cup star Jean Borotra of France and his personal trainer and friend Baron Gottfried von Cramm of Germany, who had been imprisoned by the Nazi Government on the charge of a homosexual relationship with a Jew.

[14] In 2021 the alleged events surrounding the Haijby Affair were adapted into a fictional miniseries for Sveriges Television called En Kunglig Affär (A Royal Secret), directed by Lisa James Larsson and written by Bengt Braskered.