The Battle of Sakarya 1921 was an important engagement in the Greco-Turkish War (1919-1922).

The 1920 Treaty of Sèvres, which ended the First World War in Asia Minor divided the Ottoman Empire, assigned all these territories to Greece. Greek troops had already occupied Smyrna in May 1919. Meanwhile the former Ottoman general Mustafa Kemal (later Kemal Atatürk) had formed a new Turkish nationalist government at Ankara, which repudiated the Treaty of Sèvres and prepared for war against the Greeks.

In October 1920, with the encouragement of Lloyd George, the Greek army advanced into Anatolia with the intention of defeating the Kemalist forces before they were ready to attack the Greek perimeter at Smyrna. This advance was begun under the Liberal government of Eleftherios Venizelos, but soon after the offensive began Venizelos fell from power and was replaced by Dimitrios Gounaris, who appointed inexperienced monarchist officers to senior commands. King Constantine took personal command of the army at Smyrna.

In June 1921 the Greek army advanced to the River Sakarya , less than 100km west of Ankara. But in August Kemal counter-attacked, routing the Greeks at the Battle of Sakarya (August 23 - September 13, 1921). The Greeks retreated in good order and still hoped to defend their base at Smyrna. They appealed to the Allies for help, but early in 1922 Britain, France and Italy decided that the Treaty of Sèvres could not be enforced and should be revised.

In March 1922 the Allies proposed a ceasefire, but Kemal said there could be no settlement while the Greeks remained in Anatolia. In August the Turks launched a new offensive, defeating the Greeks at the Battle of Dumlupinar near Afyon (August 30, 1922). Shortly after (August 30) the Turks captured Smyrna. Fleeing Greeks burned the city to the ground and all the Greeks who cooperated with the invaders were punished. However, this has been denied by the Greeks, who claim that the Turkish force burned the city of Smyrna, and massacred any Greeks which could be found.

Mustafa Kemal refused to walk on Greek flag on the way to the city hall despite the Greek commander invading Izmir did walk on Turkish flag.