[bergie]

Istanbul is a big city, but is not the capital. It was the capital of the Ottoman Empire, and before that the East Roman Empire, though.

[pfannkuchen]

Too close to the border?

[em-bee]

constantinople was occupied by the allies after WWI, causing the turkish resistance to set up headquarters in angora which was later renamed to ankara. after the war of independence they decided to make ankara the capital of the new republic

[esafak]

The name Ankara was in use by the Seljuks and then the Ottomans since the 16th century, according to Wikipedia:

https://tr.wikipedia.org/wiki/Ankara_(isim)