With the borders secured with treaties and agreements at east and south, Mustafa Kemal was now in a commanding position. The Nationalists were then able to demand on 5 September 1922 that the Greeks evacuate East Thrace, Imbros and Tenedos as well as Asia Minor, and the Maritsa  River should again become the western border of Turkey, as before 1914. The British were prepared to defend the neutral zone of Constantinople and the Straits and the French asked Kemal to respect it, to which he agreed on 28 September. However, France, Italy, Yugoslavia, and the British Dominions objected to a new war. France, Italy and Britain called on Mustafa Kemal to enter into cease-fire negotiations. In return, on 29 September Kemal asked for the negotiations to be started at Mudanya. Negotiations at Mudanya began on 3 October and it was concluded with the Armistice of Mudanya. This was agreed on 11 October, two hours before the British intended to engage at Chanak, and signed the next day. The Greeks initially refused to agree but did so on 13 October. Factors persuading Turkey to sign may have included the arrival of British reinforcements. The armistice then made it possible for the allies to recognise the Turkish claim to East Thrace, which was agreed to at the Lausanne Conference on 20 November 1922.
Question: What formed the western border of Turkey before 1914?
Answer: Maritsa River