I would argue that there were two broad causes for the decline of the Ottoman Empire.
The first of these was an internal cause, namely, that the Empire was too big and too diverse to be ruled easily. The Empire was made up of so many different nationalities, many of whom wanted self-rule, that it was very difficult to keep it together.
The second type of cause was external. The Ottomans were pressured, for example, by Russian expansion and later by the British and French during WWI. This external pressure often exploited the internal splits in the Empire as well.
For these reasons, the Ottoman Empire declined and fell apart in the early 20th century.
Weak and incompetent sultans
Among all of the internal reasons mentioned, incompetent sultans were the main reason of the empire’s decline. The first weak sultan was Selim II (1566-74). He was the successor to the Suleiman, the Grand Turk. This was the first weak sultan of the next 24 ones, which would lead the empire to its slow decline. Selim was the first sultan to drink wine. Selim had a poor figure to be a sultan as his father Suleiman didn’t give him the training he needed to become a strong sultan. As to become a sultan the prince should serve in military or government campaigns. Indolent and dissolute, he won little respect from his ministers and subjects. At his time, the ottomans had their first defeat in battle at Lepanto. This was the first time the western world believed that the Turks could be defeated.
Corrupt executive and judicial officers
Weak of the sultans meant weak and corrupt executive officers. Selim II’s son Murad can be an example for this. He was weak as his father. He loved both gold and women. He never really cared about the empire’s affairs. He surrounded himself with numerous courtiers, which none of them were faithful. Of course there were some of who were sincere with their work, but the other viziers and grand viziers didn’t like them, because mainly those were the ones which the sultans gave his most attention and listening to. This made other people jealous. So they started to make plans of how to get rid of them, and sometimes this meant assassination.
We can take as an example of this the grand vizier Sokullo. When sultan Selim II was on the throne, he gave Sokullo a big authority, which mainly stabled the empire during selim’s reign. After Selim died, his son Murad was on the throne. Sokullo was still a grand vizier, sokullo didn’t enjoy the full authority and power that Selim has given him. Many hated him and were plotting against him. Finally, Sokullo was assassinated in his council chamber by and apparent suppliant dervish who stabbed a dagger in to his heart killing him. Corrupt officers meant that they would do anything to keep their high positions. So they used to bribe officials.
Also inheriting of positions was also introduced for the first time. Before officers were chosen by their quality.
Soon officials of the judicial systems (judges) also followed in to the corruption system. Judges were as corrupt as other officials too. Instead of listeninig to the complaints of the villagers, they were all too welcome to take bribes from those charged with offenses.
Corruption and weakness in the officers of the military forces
Corruption and weakness in the officers of the military forces also had a significant effect on the empire. This was mainly in the janissary officers and soldiers. Sultan Murad I of the Ottoman Empire founded the units around 1365. They were mainly formed from Christian youths and war prisoners from the Balkans. They were distinguished from other soldiers by their marching music and their regular salary. By time through their attachment to the sultan they started to gain too much power, and by power corruption appeared too. Also since Suleiman’s reign janissaries were allowed to marry which led to the growth of heredity among them.
Revolt of the Janissaries