As a soldier, and subsequently Maestre de campo of the Tercios, he was posted to Sicily, Africa, Malta, Corsica, Milan, the Netherlands, Spain, Portugal, France and Ireland, where he participated in major military events of his time, such as the Siege of Malta, the Looting of Antwerp, the Siege of Antwerp, the "Miracle of Empel", an expedition in support of the French Catholics during the French Wars of Religion, the Raid on Mount's Bay and another one in support of Irish clans during the Nine Years War.

He was the fourth son of Miguel del Aguila y Velasco and Sancha de Arellano, and grandson of Lord Villaviciosa.

In 1563, at eighteen years old, Juan Del Águila followed the Gonzalo de Bracamonte's company which joined the Sicilian Tercio.

In 1568 started the Eighty Years' War also called the Flanders' revolt against the Spanish Monarchy, which it would end in 1648 with the Treaty of Westphalia In 1569 Captain Pedro Gonzalez de Mendoza promoted Águila to lieutenant.

In February 1580, the governor Alexander Farnese (Juan de Austria had died in 1578) was forced to repatriate the Tercios because of the negotiations for submission by the Walloons.

Three months later, on 23 July, the major city Nieuwpoort surrendered to the Spanish and del Águila became the new governor, and his company became the garrison.

At the end of 1584 Siege of Antwerp began, in which Juan de Águila and his Tercio particularly distinguished themselves by defeating the Dutch who tried to rescue the city from the Covenstein dyke (27 May 1585).

After taking Antwerp, Farnese graduated a part of the army and sent the rest to the north to help reimpose Catholicism and Spanish rule in the northern Netherlands.

With the artillery that they had managed to salvage, the troops of Juan del Águila occupied an islet that had formed after the flood and staved off the rebel boats to prevent them from approaching.

Meanwhile, the Dutch seized other islets and began construction of fortifications, which ended in record time despite the Spanish cannon shots.

On the night of 7 December a soldier found, buried near the church of Empel, a table with the image of the Immaculada and, as 8 December was its feastday, the discovery was considered a good omen: This very rich treasure that they discovered under the earth was a divine proclamation of good fortunes, and through the intercession of the Virgin Mary, they were waiting the blessed day[2]That same night, Bobadilla ordered an assault on the forts with the few boats available.

On 24 January 1586, Don Juan D'Aguila and his troops, including 17 foot soldiers and 4 Cornet cavalry horsemen, raided the German villages of Boslar, Muntz and Gevenich.

[3] Later in 1586, Juan del Águila's Tercio participated in the conquests of Grave (6 June), Neuss (26 July), Alpen (13 August) and the rescue of Zutphen (22 September), forcing the English army that had besieged it to lift the siege.

Ten days later they embarked again, this time with orders to escort the Indian fleet in the final stretch of its journey to Lisbon.

In 1593 part of the Juan del Águila army landed on Camaret and built "La Pointe des Espagnols" (The Tip of the Spaniards) fort in the Crozon peninsula, dominating the entrance to the Brest port.

On 1 October, an Anglo-French army started a siege of Fort Crozon, while an English fleet bombarded the place from the sea.

The garrison could only hold out until 15 November, while the auxiliary army, led by Juan del Águila, failed to relieve the fort having been blocked at Plomodiern.

Taking advantage of the "break" that he got from the French troops, Juan del Águila decided to organize a punitive expedition against England, for helping Henry IV of France.

In May 1600, Juan del Águila was imprisoned for "having taken advantage of the tax of the king more than what was just," as reported by Luis Cabrera de Córdoba.

Thus, most men were able to land on Irish soil, but eight or nine ships under Pedro de Zubiaur, along with 650 soldiers and most of the provisions, returned to Galicia.

Seeking to rescue Juan del Águila's forces and break the siege, the anti-Tudor coalition of the Irish clans of Ulster under the leadership of Aodh Mór Ó Néill and Red Hugh O'Donnell marched across the whole length of Ireland through extremely bitter winter conditions with 5,500 men.

In Spain, Pedro de Zabiaur, left port on 7 December in command of ten ships with 829 soldiers and abundant provisions and ammunition.

They managed to destroy twenty guns and inflicted hundreds of casualties on the besiegers, but they had to return to the city, unable to cross English lines.

[citation needed] A rebel army coming from the north, finally managed to link with the Spanish, then decided to help Juan del Águila.

[citation needed] At that time, the English army had been reduced to 8,000 men due to battle casualties, diseases and desertions.

The lack of coordination between the rescue army and the besieged, coupled with the disorganization of the Irish and the superiority of the English cavalry, turned the Battle of Kinsale into a major defeat for the Spanish-Irish coalition.

[citation needed] On 14 January, just two days later, Martín de Vallecina arrived in Kinsale with reinforcements, but returned to Spain as soon as he learned of the surrender.

According to Emilio González López's account: "Overwhelmed by this arrest, which involved a severe censure of his military conduct in Ireland, Don Juan del Aguila died, probably in early August.

However, the impossibility of him telling his version of the events caused the story to reach the Court via the hands of others who were not involved, and his subsequent death made the Council decide without Juan del Águila's testimony and without him relating his defence of Kinsale for three months against superior forces.