The Uskok War, also known as War of Gradisca, was fought between the Austrian and Spanish on one side and the Venetians, Dutch and English on the other. It is named for the Croatian soldiers known as Uskoks used by the Austrians for guerrilla warfare.
Since the Uskoks were checked on land and were rarely paid their annual subsidy, they resorted to acts of piracy. Uskoks did not limit their attacks to Turkish vessels but also Venetian merchantmen were often attacked by the Uskoks (and later also attacked by Spanish corsairs). The Venetians tried to protect their shipping with escorts, watchtowers, and other protective measures, but the costs became too high: 120,000 Thalers annually in the 1590s, 200,000 in the 17th century, and 360,000 by 1615. In December 1615, Venetian troops laid siege to Gradisca, located on the Isonzo River.
The Venetians launched a major diplomatic campaign for allies, since the Uskoks were vassals of then-Archduke Ferdinand of Inner Austria, who was likely to seek help from the Holy Roman Emperor Matthias (his uncle) and King Philip III of Spain (his brother-in-law). In September 1616, Count John Ernest of Nassau-Siegen agreed to raise 3,000 men in the Dutch Republic for Venetian service. They arrived the next May, followed six months later by another 2,000 with a contingent of English volunteers. Support from the Spanish was prevented on sea by a flotilla of 12 Dutch and 10 English warships, and on land by the war in Mantua.