John became a military leader in the service of his half-brother, King Philip II of Spain, Charles V's heir, and was addressed to as a Don.

[1] Charles V wrote a codicil, dated 6 June 1554, in which he recognized: "For since I was in Germany, after being widowed, I had a natural child of one unmarried woman, named Geronimo".

Rumors had spread about John's paternity, which de Quijada denied, and he wrote to Charles asking for instructions.

Charles replied with a note written by his personal secretary Eraso, in which he recommended to wait for Philip's return to Spain.

Philip insisted that, although John was a member of the House of Habsburg, he was not to be addressed as "Your Highness", the form reserved for royals and sovereign princes.

John was known as "Don Juan de Austria", and his manner of address was "Your Excellency", the title used for a Spanish grandee.

[3][5] John completed his education at the University of Alcalá de Henares (now the Complutense University of Madrid), where he attended with his two nephews: Prince Carlos, son and heir of Philip II, and Alessandro Farnese, the son of Charles V's other acknowledged illegitimate child, Margaret of Austria, Duchess of Parma.

[3][5] John actively participated in court ceremonies, including at the baptisms of his nieces, Isabella Clara Eugenia and Catherine Michaela.

John asked Philip for permission to join the navy, but was denied; he then left the court and travelled to Barcelona, but was unable to reach the fleet in time.

John continued his attempts to join the fleet of García Álvarez de Toledo y Osorio until a letter from Philip caused him to give up.

[citation needed] Prince Carlos confided in John and told him of his plans to flee Spain and travel towards the Spanish Netherlands from Italy.

[citation needed] A decree dated 1 January 1567 forced the Moriscos who lived in the Kingdom of Granada, particularly in the Alpujarras region, to abandon their customs, language, clothing, and religious practices.

Philip deposed Iñigo López de Mendoza, 3rd Marquis of Mondejar, and appointed John the supreme commander of the royal forces.

The siege of Galera stalled, so John ordered a general assault, making use of artillery and strategically set mines.

[6][7] When he marched on the fortress of Serón, John was shot in the head, and his foster father Luis de Quijada was wounded.

[citation needed] The War of Cyprus became the focus of Spanish attention after Pope Pius V sent an envoy to urge Philip to join him and Venice in a Holy League against the Turks.

He agreed that Cyprus should be protected, but also wished to recover control of Tunis, where the Turks had overthrown Philip's Muslim client ruler.

On reaching the Turkish coast in September, Colonna and the Venetians wished to continue towards Cyprus while Doria argued that it was too late.

He travelled with the Spanish squadron from Barcelona in July, and the Holy League armada set sail from Messina in mid-September.

[3][5] When Luis de Requesens died on 5 March 1576, the Council of State urged the king to appoint a new governor immediately, recommending that it be a member of the royal family.

The victory of John also meant the end of the Union of Brussels, and hastened the disintegration of the unity of the rebel provinces.

To avoid his body being captured at sea by Spain's enemies, it was returned to Madrid overland in four saddle bags and reassembled once there.

[12] As John of Austria was the illegitimate son of Charles V, the partitions of the armories of his father's coat of arms were modified for his own.

John's coat of arms consisted of a divided shield; on the dexter (left) side, the arms of Castile and León were placed in a cut and not quartered (repeated in four quarters), as usual; on the sinister (right) side were Aragon and Aragon-Sicily; and on the divided inescutcheon (center) were Austria and Duchy of Burgundy.