Ankara replaced Istanbul as the capital of Turkey in 1923. The change marked the birth of the modern Turkish Republic and symbolized a political break with the Ottoman imperial past.
During the Turkish War of Independence, Ankara served as the centre of the nationalist movement led by Mustafa Kemal Atatürk. Its inland location in central Anatolia was easier for the movement to defend and placed the new government closer to the heart of the territory it controlled. The Grand National Assembly established its government in Ankara in 1920.
The Republic of Turkey formally designated Ankara as its capital on 13 October 1923, shortly before the republic was proclaimed on 29 October. Istanbul remained Turkey’s largest and most internationally famous city, but it was no longer the political capital. Bursa, Izmir, and Istanbul are therefore historical or major cities, not the answer to this question.