The Greco-Turkish War of 1897 or the Ottoman-Greek War of 1897 (Turkish: 1897 Osmanlı-Yunan Savaşı or 1897 Türk-Yunan Savaşı), also called the Thirty Days' War and known in Greece as the Black '97 (Greek: Μαύρο '97, Mauro '97) or the Unfortunate War (Greek: Ατυχής πόλεμος, romanized: Atychis polemos), was a war fought between the Kingdom of Greece and the Ottoman Empire. Its immediate cause involved the status of the Ottoman province of Crete, whose Greek-majority population had long desired union with Greece. Despite the Ottoman victory on the field, an autonomous Cretan State under Ottoman suzerainty was established the following year (as a result of the intervention of the Great Powers after the war), with Prince George of Greece and Denmark as its first High Commissioner.

The war put the military and political personnel of Greece to test in an official open war for the first time since the Greek War of Independence in 1821. For the Ottoman Empire, this was also the first war-effort to test a re-organized military system. The Ottoman army operated under the guidance of a German military mission led (1883–1895) by Colmar Freiherr von der Goltz, who had reorganized the Ottoman military after its defeat in the Russo-Turkish War of 1877–1878.

The conflict proved that Greece was wholly unprepared for war. Plans, fortifications and weapons were non-existent, the mass of the officer corps was unsuited to its tasks, and training was inadequate. As a result, the numerically superior, better-organized, -equipped and -led Ottoman forces, heavily composed of Albanian warriors with combat experience, pushed the Greek forces south out of Thessaly and threatened Athens,

only to cease fire when the Great Powers persuaded the Sultan to agree to an armistice.