Many empires in human history "seem to have come from nowhere", and the Ottomans are no exception, but certain factors seem to stand out as possible causes (Montreal Review). These include:
-A Power Vacuum. The collapse of the Seljuk Turks, combined with the decline of Byzantium, left the Middle East free of a dominant power. The Ottoman empire rose, in part, to bridge the gap between the great powers of China and Europe.
-Reaction to Crisis: A crisis, in this case, is defined as "a general, sometimes abrupt worsening of economic, political, and social conditions, carrying with it a sense of impending change" (Di Cosmo). The aforementioned collapse and the situation of warring gazis caused fear and made people more vulnerable to the call of a rising leader.
-Militarization: a state of crisis meant that pastoral societies had to maintain standing armies. In such situations, military leaders abounded. Militarization also indicates a centralization of government, the creation of imperial guard units, and the general cohesion of a military-dominated society.
-The Charismatic Leader: Pastoral societies organized around a powerful leader, in this case Osman I.
-Centralized Government: Having chosen a leader around whom to center power, the Ottomans were able to centralize the military power structure and create localized authorities to complement it.
-Revenue and Territorial Expansion: Collecting tribute from subjects was supported by tradition, and a centralized state allowed more a more systematic collection of taxes. More funding fed and clothes the expansive armies already in place, which enabled expansion far beyond the original borders of the Turks' homeland all the way to Europe.
The effective military and bureaucratic structures of the previous century came under strain during a protracted period of misrule by weak Sultans. The Ottomans gradually fell behind the Europeans in military technology as the innovation that fed the Empire's forceful expansion became stifled by growing religious and intellectual conservatism. But in spite of these difficulties, the Empire remained a major expansionist power until the Battle of Vienna in 1683, which marked the end of Ottoman expansion into Europe. Various instances began to wear away at the Ottomans' influence, including a naval defeat by the Spanish which destroyed the image of naval supremacy and the discovery of new maritime trade routes which allowed Europeans to bypass the ottomans in trading with China. To Ottomans, " decline ” meant dislocation of the traditional order; hence, ” reforms " to check or reverse " decline " meant restoring the old order which had produced the Golden Age of Suleyman the Magnificent.” At times decline was checked but only temporarily. Decline was not only slow, gradual, interrupted, lasting rnore than three centuries, but also it was relative only to its own Golden Age and to the remarkable progress of its Christian European neighbors.
It was during this period that the Ottoman Empire was widely known as "The Sick Man of Europe".
Reasons behind the Decline and Fall of the Ottomans:
-Colonialism: While European nations were getting rich through colonialism and the trade of goods they plundered there, the Ottomans could not expand and were preoccupied with their existing territories.
-Technology; While Europe was advancing technologically, Ottomans were left behind. The scientific mindset of Europe at this time encouraged development, while the more traditionalist philosophy of the audience kept them back.
-Over-Extension: Ottomans had a very wide expanse of territories to protect, so when localized rebellions started, with the provocation and the support of various European countries, the Ottomans lacked the money and military power to quell them all.
-Commercial Efforts: Europe was more successful economically, due to inflation, new trade routes that bypassed the Ottoman monopoly, and scientific advances in many areas.
-Competition: Strong, centralized, national monarchies or bureaucratic empires appeared not only in Western Europe but also along the Ottoman frontiers in Central and Eastern Europe just when centrifugal forces were weakening the previously centralized Ottoman bureaucratic empire.
-Lack of Middle Class Growth: While Europe's middle class was wildly successful in pushing economic and political development, the wealthy bourgeoisie which did exist in the Ottoman Empire was small and composed largely of either non-Muslim merchants and bankers, who were not acceptable as the sultan’s allies, or bureaucrats, who were a part of the "establishment ” anxious to protect their own interests and often resisting change.
-Trouble at Home: The Ottomans were concerned with their own problems of weak leadership, bribery and favoritism, a dissolving military heirarchy, severe inflation due to the silver drain, and a decline of intellectual advancement.
-Expansion Stopped: Expansion was a part of Ottoman culture from the beginning. When expansion stopped, the collective psyche of the people began to decay.
-Supposed Superiority: Similarly to Ming China, some experts say that the Ottomans were so firmly convinced of their own cultural superiority that they simply got lazy, forgetting to maintain what they had achieved.
-Too Many Autonomous Groups: There were many groups within the multicultural empire that had been allowed a great deal of autonomy throughout the years. Never had they been fully integrated into the Ottoman mindset; when they began to revolt, and territories began to leave the empire in their quest for political freedom, the failing state was unable to keep hold of them.
-World War I: Unfortunately, the Ottomans chose to support the losing side. The final end of the Ottomans was the Treaty of Versailles, when the Ottoman Empire was divided up by the victorious powers into areas of economic interest.
These dates are a compromise based on all available sources.
Other relevant Wikipedia articles/supporting sources
Osman I (Not to be confused with Otto I, ruler of the Holy Roman Empire from 936-973) was a gazi leader who established a sizable realm in Western Anatolia. This was the true birth of the Ottoman Empire, with Osman I being credited as the eponymous founder of the ruling family.
The Empire began to spread, crossing into the Balkans in 1345 as an ally of the Byzantine Emperor to defeat the Serbs. After being called upon a second time, the Ottomans remained in Byzantium, setting up a base at Gallipoli. After failing to gain help from the Bulgars or Serbs to expel the Ottomans, the Byzantine Emperor abdicated, allowing the Ottomans to gain a further reach.
During this period, the Ottomans succeeded in subjugating Bulgaria, Serbia, Greece, and much of Albania, building a fairly stable kingdom in the Balkans. After the death in captivity of Sultan Bayezid, in 1403, the kingdom dissolved into a ten-year dynastic struggle.
After the state was restored by Mehmet, the Ottomans regained much of their lost territory and expanded even further, fighting wars with Venice and provoking further crusades against them from Europe. In 1451, Mehmet II, known as the Conqueror, succeeded Murat II and launched a period of aggressive and successful territorial expansion; they take Bosnia, Albania, and several other small European territories.
This was a turning point in the conquering of the Ottoman Empire, because it removed the final Middle Eastern obstacle to the Ottomans becoming the dominant power of the region. The Orthodox people of Constantinople, due to bad blood with Western Europe, accepted their Islamic rulers as preferable to Venetian ones.
Sultan Selim I dramatically expanded the Empire's eastern and southern frontiers by defeating Safavid Persia, in the Battle of Chaldiran. Selim I established Ottoman rule in Egypt, and created a naval presence on the Red Sea. After this Ottoman expansion, a competition started between the Portuguese Empire and the Ottoman Empire to become the dominant power in the region. The Ottoman Empire flourished economically due to its control of the major overland trade routes between Europe and Asia.
Suleiman the Magnificent captured Belgrade in 1521, conquered the southern and central parts of the Kingdom of Hungary as part of the Ottoman–Hungarian Wars, and established Turkish rule in the territory of present-day Hungary (except the western part) and other Central European territories. He then laid siege to Vienna in 1529 and again in 1532, but was unsuccessful both times. Transylvania, Wallachia and, intermittently, Moldavia, became tributary principalities of the Ottoman Empire. In the east, the Ottoman Turks took Baghdad from the Persians in 1535, gaining control of Mesopotamia and naval access to the Persian Gulf.
France and the Ottoman Empire, united by mutual opposition to Habsburg rule, became strong allies. The French conquests of Nice (1543) and Corsica (1553) occurred as a joint venture between the forces of the French king Francis I and Suleiman. After further advances by the Turks in 1543, the Habsburg ruler Ferdinand officially recognized Ottoman ascendancy in Hungary in 1547.
By the end of Suleiman's reign, the Empire's population totaled about 15,000,000 people extending over three continents.  Plus, the Empire became a dominant naval force, controlling much of the Mediterranean Sea. By this time, the Ottoman Empire was a major part of the European political sphere.
This was a near 130-year period during the 16th and 17th centuries when the women of the Imperial Harem of the Ottoman Empire exerted extraordinary political influence. Many of the Sultans during this time were minors and it was their mothers, leaders of the Harem, who effectively ruled the Empire. Most of these women were of slave origin, due to the need for the House of Osman to maintain its prestige; no other royal house was perceived as being prominent enough to be worth marrying into.
The Tanzimat, meaning “reorganization” in Turkish, was a series of reforms undertaken under the rule of Sultan Abd al-Majid in order to modernize society along secular and bureaucratic lines. The first set of reforms, expressed by the 1839 edict entitled Hatti-i Sharif, sought to secularize the government's treatment of people and property and to reform taxation and military conscription. Foremost among the laws was the security of honor, life, and property for all Ottoman subjects, regardless of race or religion, which was modeled after Western philosophy of the day. Later reforms (1856) included the attempted reduction of theological dominance, establishment of a secular school system and a new law code. the lifting of monopolies, fairer taxation, secularized schools, a changed judicial system, and new rules regarding military service. Efforts to centralize government administration, however, ended by concentrating all authority in the hands of the sultan Abd al-Hamid II, who often abused the power. The constitution of 1876, while it promised the democratic reforms of a Constitution and a parliament, actually was intended to stave off European intervention. This ended the Tanzimat era.
This group was dissatisfied with Tanzimat reforms and sought to transform the Ottoman society by preserving the empire, revitalizing Islam and modernizing along the European traditions. Many members of the society were writers, and they attempted to spread their ideas through several publications. Ultimately, their ideas led to a very brief period of Constitutional monarchy, after which the group fell below the national radar.
This was a brief period of constitutional monarchy based around a constitution written by members of the Young Ottomans. No party system developed, but the parliament was seen as the voice of the people. The era ended with the suspension of Parliament by Abdülhamid II.
The Young Turks were a Turkish nationalist reform group who favored the re-establishment of a secular Constitutional monarchy in the Ottoman Empire. They began as an intellectual group stemming from the failure of the young Ottomans to bring about positive reform; after official formation, they worked under the umbrella of the Committee of Union and Progress (CUP). They were successful in their revolt, and their ideas brought about the Second Constitutional Era, which remained in place until the end of WWI.
World War I was the event that finalized the downfall of the Ottoman Empire. Because of past loyalties, the Ottomans forged a secret alliance with Germany in 1914. The ottoman army mobilized quickly, warranting a Declaration of War from Russia. In the winter of 1914-15, a staggering defeat by the Russians caused the demoralized army to turn their blame on the Armenian minority in the Ottoman Empire. The slaughter that occurred turned Western powers against the Ottomans (see "Armenian Genocide" heading for more on this topic). In the following years, General Ataturk earned the respect of the Turkish people by successfully defending Turkey against many attacks, on both land and water; he also withdrew several units intact from Syria in order to preserve the fighting force. Eventually, after several months of fighting in Egypt and Palestine, the Ottoman defense was exhausted. The war-time government resigned, and the new Sultan signed the armistice in October of 1918. In four years of war, the Ottoman Empire had mobilized about 2.8 million men, of whom about 325,000 were killed in battle. In addition, more than 2 million civilians, including both Turks and Armenians, are believed to have died of war-related causes. The Ottoman Empire collapsed and was partitioned in the Treaty of Versailles.
Brought about by suspicions of treason during World War I, the Armenian Genocide was the Ottoman government's systematic extermination of its minority Armenian subjects from their historic homeland in the territory constituting the present-day Republic of Turkey. It took place during and after World War I and was implemented in two phases: the wholesale killing of the able-bodied male population through massacre and forced labor, and the deportation of women, children, the elderly and infirm on death marches to the Syrian Desert. The total number of people killed as a result has been estimated at between 1 and 1.5 million. The Assyrians, the Greeks and other minority groups were similarly targeted for extermination by the Ottoman government, and their treatment is considered by many historians to be part of the same genocidal policy. It is acknowledged to have been one of the first modern genocides.
This is the first appearance of the Turks in Chinese records; the Turks are referred to as the "Hsiung-nu" an early form of the term "Hun". (Yes, like in Mulan, though they are a few centuries out of place in that case.) These people " lived in an area bounded by the Altai Mountains, Lake Baikal, and the northern edge of the Gobi Desert and are believed to have been the ancestors of the Turks" (Pitman).
Tu-Kue is mentioned in Chinese records as having been a tribal kingdom located on the shores of Orkhon River in modern-day Mongolia. The Khans of Tu-Kue accepted the cultural and economic superiority of Tang China.
This was the first sample of Turkic writing was found in the area of what once was Tu-Küe. It was dated to around this time.
The Oguz Turks migrated from Central Asia during the first millennium AD, along with several other tribes. The Oguz converted to Islam in the 10th century and established themselves under their Khan, Seljuk. Descendants of Seljuk led their people west, and they conquered Baghdad in 1055 with an army of gazis (soldiers of the Islamic faith). The caliph of Baghdad eventually accepted the Seljuk leader as Sultan. In the Battle of Manzikert (1071), the Seljuks conquered both Christians and Greeks to take full control of Armenia and Anatolia. Over the next centuries, the Seljuks established control over much of the Middle East, becoming champions of the Sunni Muslim tradition in opposition to the Shia factions.
"The Terror of the Turk" abounded in Europe in the 11th century, mostly due to Seljuk Sultanate and its alliance with the Abbasids. The fear triggered a military response: the Crusades, a holy war against the Islamic peoples. The Turks were defeated in 1097, but they gained most of the land back in the 1140s. Through numerous attacks, the Sultanate of Rum survived until the 13th century as vassals of the Mongol Empire. When the Mongols collapsed, several gazi amirates remained, competing for supremacy. Any hopes they had, however, were dashed by the rise of the Ottomans.
The strongest and most powerful of the Seljuk sultanates, the Sultanate of Rum ("Rome", like the surviving Byzantine Empire) ruled the Seljuk empire for the rest of its existence. Rum was a tribal confederation, centered around a ruling family from whom the sultan was chosen. The sultan was primarily a military leader, and below him in importance was a heirarchy of viziers (vice-regents), amirs (governors), and begs (regional commanders). This structure did not change the local systems of governance throughout Armenia, though Turkish culture and Islam spread through the countryside by intermarriage.