Final answer:
The Ottoman Empire's decline began as early as the 16th century and was exacerbated by nationalistic movements and European military might. The partitioning followed World War I, with the Treaty of Sèvres and later the Treaty of Lausanne delineating the new national boundaries and mandates.
Step-by-step explanation:
The Ottoman Empire experienced a long decline after reaching its zenith during the 16th century. The European powers recognized this weakening empire as an opportunity to expand their own influence. The Greek War of Independence in the 1820s signaled the beginning of the end for the Ottoman territorial integrity with the successful independence of Greece. Following this, the Balkan states of Serbia, Romania, and Bulgaria also gained independence in the latter half of the 19th century. The European countries continued to encroach upon Ottoman lands, taking advantage of their weak leadership and internal strife.
During World War I, the Ottoman Empire's role was further diminished, culminating in the signing of the Armistice of Mudros in 1918. In the aftermath, the Treaty of Sèvres in 1920 proposed an almost complete division of Ottoman territories, but the Turkish National Movement led by Mustafa Kemal Atatürk opposed this division. Successful resistance by the Turks led to the revision of the terms, which were finalized in the Treaty of Lausanne in 1923, establishing the borders of modern-day Turkey and transitioning other regions into mandates under British and French control according to the League of Nations mandate system
Nationalistic uprisings had weakened the empire from within, while European military advancements and strategic interests in the region precipitated its dissolution. The aftermath of the empire's collapse laid the foundation for modern conflicts in the region, as new national boundaries did not always align with historic ethnic or sectarian lines.