Spanish Ottoman Relations from 15th to 17th Centuries

It's important to get a handle on the overall political picture in Europe at the time before diving into Spanish-Ottoman relations. Beginning in 1519, Castile and Aragon were inherited by the Duke of Burgundy (among a million connected titles), who also inherited the Habsburg hereditary lands in central Europe, and was elected Charles V, Holy Roman Emperor. Though he came to realize that one man couldn't rule all of Spain and Germany, and divided his dominions between his brother and son, it's important to remember that Spain, much of Italy, the Netherlands, and Austria and the Holy Roman Empire have the same ruling family. The House of Habsburg would develop a reputation for being hardcore Catholics in the coming centuries, and would eventually lay claim to the entirety of the New World when they inherited both Spain and Portugal. Until 1648, they were probably the most powerful dynasty on the globe.

All these dominions border France at one point or another, and the Valois royal family of France were quite concerned about being completely encircled by Habsburg lands, and in 1536 found an ally that could counterbalance Habsburg ambitions. Both France and the Ottomans had plenty to gain by supporting each other against the Habsburgs; the Ottomans could take over much of Eastern Europe at the expense of the Austrian branch, and expand their mediterranean holdings into the territory of the Spanish Habsburgs and their allies. This gave the French breathing room in which they could work to counterbalance Habsburg domination in Western Europe, which they used to prosecute the Italian Wars, which quickly spiraled out of Italy and into everywhere Habsburg and Valois dominions brushed up against each other; one of the most famous battles of the Italian Wars, Philip II of Spain's victory at St. Quentin, was fought on the border between the Spanish Netherlands and Picardy in France.

Relations between Spain and the Ottomans during the 16th century were dominated by war; both empires were continually jostling for position throughout the Mediterranean, besieging fortified naval bases, duking it out on the seas, and setting up allied states in North Africa. One of the most famous battles of the period is the Siege of Malta, an island that offered control of the Central Mediterranean, in which the Knights of St. John held out long enough for a relief force to drive away the Ottoman fleet in 1565, and six years later, Philip II's half brother, Don John of Austria, led the ships of the Holy League, an alliance of Catholic Italian states, and shattered the Ottoman navy off the western coast of Greece in the Battle of Lepanto. However, the League's primary mission had been relieving Ottoman encroachments on Venice's holdings on Cyprus, which were another key strategic point, this time for controlling the Eastern Mediterranean, though the Venetians managed to hold Crete for another century. These bases throughout the Mediterranean were key, since navies of the time were made up primarily of galleys. With shallow drafts and high crew demands stemming from the use of oars, these ships would quickly run out of stores, so possessing a chain of harbors in which to restock was critical for power projection on the seas. Both dynasties set up allied polities in North Africa as well, with Tunis being a particular focal point; ultimately, the Ottomans won out in 1574. War between the branches of the Habsburg family and the Ottomans would continue on and off for the next two hundred years or so, though the key theatre shifted to Eastern Europe, where the Austrians pushed the Turks back, reclaiming Hungary after its loss in 1526 during the Great Turkish War of 1683.

Geoffrey Parker has written extensively on Spain in the 16th Century; The Grand Strategy of Philip II and Imprudent King are two of the more well known books that touch on the topic, but this spills over into a huge range of topics and sources one can't even begin to list.