Final answer:
The Ottoman Empire's decline began post-Süleyman the Magnificent due to incompetent leadership and other issues. Losing the Battle of Lepanto and various territories, the empire tried to reform but continued declining, leading to its end after World War I and the establishment of the Republic of Turkey.
Step-by-step explanation:
The Ottoman Empire experienced a significant decline after the reign of Süleyman the Magnificent. Subsequent leaders lacked the competence required to manage such a large empire effectively, leading to periods of military corruption, governmental decentralization, and other setbacks. The Battle of Lepanto in 1571 marked a substantial military defeat, diminishing the empire's naval supremacy.
Overextended and facing emerging European powers, the empire slowly lost ground. By the end of the seventeenth century, territories such as Hungary and Transylvania were lost to the Austrians, symbolizing the empire's weakening grip.
During the nineteenth century, the empire, labeled as "the sick man of Europe," tried to implement administrative reforms and sought European military advice in an attempt to modernize and stave off decline. Despite these efforts, the empire continued to struggle with internal revolts and external pressures from European nations eager to carve up Ottoman territories. World War I further expedited its collapse, leading to the eventual fall of the empire and the birth of the modern Republic of Turkey in 1923.