In the late 1500's, the Ottoman Empire started going into decline as a result of both internal and external factors. Internally, the Ottomans suffered from three major problems. First of all, after Suleiman's death, the sultans were less capable and energetic, being raised and spending their time increasingly at court with all its harem intrigues. Without the sultan's strong hand at the helm, corruption became a major problem. Second, the Janissaries became a virtual hereditary caste, demanding increasingly more pay while they also grew soft and lazy. Finally, the size of the empire created problems. The sultan was expected to lead the army, setting out with it each spring from the capital. This meant that as the frontiers expanded, it took the army longer to reach the enemy, thus shortening the campaign season to the point where it was very hard to conquer new lands. This especially hurt the Turks at the siege of Vienna in 1529. They did not reach the city until September, and winter set in early with disastrous results for the troops not used to European winters. Because of these factors, the Turks made few new conquests after 1565 and, as a result, gained no significant new revenues and plunder.
Two external economic factors also hurt the Ottomans, both of them stemming from the Age of Exploration then taking place. For one thing, the Portuguese circumnavigation around Africa to India had opened a new spice route to Asia. Therefore, the Turks lost their monopoly on the spice trade going to Europe, which cost them a good deal of much needed money. The other problem came from the Spanish Empire in the Americas that was bringing a huge influx of gold and silver to Europe. This triggered rampant inflation during the 1500’s, which worked its way eastward into the Ottoman Empire. This inflation, combined with the other factors hurting the empire's revenues, led to serious economic decline.
That economic decline hurt the empire militarily in two ways that fed back into further economic decline. First of all, after 1600, the Turks lost their technological and military edge. While European armies were constantly upgrading their artillery and firearms, the Ottomans let theirs stagnate, thus putting them at a disadvantage against their enemies. Also, as Turkish conquests ground to a halt, a stable frontier guarded by expensive fortresses evolved, which drained the empire of even more money. At the same time, Europeans were reviving the Roman concept of strict drill and discipline to create much more efficient and reliable armies. However, the Turks failed to adapt these techniques and, as a result, found themselves increasingly at a disadvantage when fighting against European armies.
Second, the tough feudal Turkish cavalry that had been the backbone of the army in the mobile wars of conquest were less useful to the sultans who now needed professional garrisons to run the frontier forts. Without wars of conquest to occupy and enrich them, they became restless and troublesome to the central government. That combined with the problems from the Janissaries, caused revolts that further disrupted the empire. (Eventually, the Janissaries would become so troublesome that one sultan would have to surround and massacre them.) Both of these military problems, the failure to keep up with the West and the increasingly rebellious army, fed back into the empire's economic decline, which further aggravated its military problems.
The following centuries saw the Ottoman Empire suffer from steady political and economic decay. By the 1800's, its decrepit condition would earn it the uncomplimentary title of "The Sick Man of Europe". Finally, the shock of World War I would destroy the Ottoman Empire once and for all, breaking it into what have become such Middle Eastern nations as Turkey, Iraq, Saudi Arabia, Jordan, Lebanon, and Israel.