The "Sick Man of Europe" was a phrase coined by Tsar Nicolas I, describing the situation surrounding the crumbling Ottoman Empire at the end of the 19th century. After their apex in the early 16th century, the Ottomans began a slow and steady decline, which culminated with this term, and with the situation the Ottoman Empire faced prior to WWI. Their relationship with Europe in this period truly began in 1453 when their conquest of Constantinople initiated, to some extent, the Renaissance, and the Ottomans failed to benefit from its influence. Unfortunately for the Ottomans, they inherited the same problems that crippled the Eastern Roman Empire before it: nepotism, corruption, a politically strong and active clergy, ethnic disunity, unruly feudal lords, and a rebellious military. So, while Europe moved through the societal and technological gains that would culminate with European imperialism, the Ottoman Empire remained relatively socio-politically stagnant. It was at this point, at the height of their power, that the Ottomans began enacting a series of trade agreements with the European powers giving them relative autonomy in certain regions of the Ottoman Empire in order to conduct trade. It granted them legal and administrative rights, and granted the various ethnic and religious groups (such as the Maronites or the Jews) protection under their respective European patrons. The Ottoman Empire, being situated on the great trade routes to Asia, depended on trade and thus used these capitulations as ways to encourage the European states to trade. But the power of the Empire began to wane. The European states, wanting to circumvent the Ottoman trade hegemony using the technology that came out of the Renaissance, began to colonize far flung continents and find alternative trade routes around Africa. When this happened, the Ottoman economy began to decline, though their administrative costs continued to rise. The Ottomans could no longer keep up with the European powers, which began to show with their defeats in the Great Turkish War. This military and technological weakness began to become apparent with their failure to prevent rebel factions from reclaiming parts of their Empire (for example, the Greek War of Independence [1821] and the Second Serbian Uprising [1815]). But it was the Ottoman Empire's failure to industrialize (both militarily and economically) which truly highlighted their weakened state. Once their monopoly over Asian trade began to subside, the Ottoman Empire turned to local, specialized industries in order to buoy their economy. This created a strong, politically active artisan class which opposed the industrialization of the Empire, something they felt would threaten their way of life. Moreover, the devshirme, who were ex-Christian soldiers (Janissaries) and state officials that were raised by the state, played a large role in the forestalling of modernization. Economically, the devshirme made sure to undermine any official who would weaken their position via industrialization. Militarily, the Janissaries actively overthrew Sultans who sought to modernize or disband them. All of these elements ensured that the Ottomans had failed to keep up militarily, economically, and technologically with the European powers.
But the European powers realized that the conquest of the Ottoman Empire would have two results: firstly, it would upset the Metternichian balance of power that had kept Europe relatively stable for decades, and secondly, the Europeans realized that the Ottoman bureaucracy was specifically designed to deal with the tribal, religious, and ethnic tensions that plagued the region. So why conquer something militarily when you can conquer something financially? The previous capitulations were used as economic wedges by which the Europeans could begin to dominate the Ottoman economy without having to conquer it. The Ottoman rulers, meanwhile, began desperately trying to modernize. They attempted European-style political reforms (Tanzimat), attempted to educate the political elite at European schools, and attempted to industrialize. Unfortunately, this cost the Empire even more capital, capital that had been damaged by their loss of trade, by their multi-front wars and rebellions, and by their domination by the European powers. The Empire began borrowing money from the European powers to support their development, which resulted in massive debt, eventually culminating with the Ottoman Public Debt Administration (1881), a European commission which began to run the Ottoman economy for them in an attempt to pay back their debt. By this point in history, the Ottomans were more than a "Sick Man of Europe," they were the "Dying Man of Europe." The glory of the Empire was long since over.
I suppose I could source the Tanzimat, or perhaps Al-Afghani on the perils of Western Imperialism and the need for unification.