With the incorporation of Hatay, which had been separated from Turkey at the end of World War I, into Turkey in 1939, the most significant border change in the history of the Republic took place. With the slogan, "The Turkish homeland of forty centuries cannot remain captive in the hands of enemies," this border change, which was determined by Atatürk himself and took place as a result of an international struggle that lasted for approximately twenty years, is one of the greatest achievements of Atatürk's foreign policy. Although he did not live to see the outcome, Atatürk's determination and resolve regarding the Hatay (Sanjak) issue played an extremely effective role in determining the matter. This process, in a sense, involved an assessment of political developments in Europe, particularly in the period immediately preceding World War II. Hatay's incorporation into Turkey took place after years of French rule, a brief period of Syrian administration, and another brief period of independence.

Hatay, which was within the borders of the Turkish National Oath, was occupied by the French along with the regions of Urfa, Adana, Antep, and Mersin, citing Article 7 of the treaty as justification. After the people's resistance and struggle for liberation, France was unable to maintain its hold on the region and was forced to sign the Ankara Agreement with the Turkish Grand National Assembly Government on 20 October 1921. According to the treaty, France withdrew from the territories it had occupied outside Hatay, while Hatay, where the majority of the population was Turkish, was left under French mandate in Syria, with the condition that Turkish would be the official language and that every effort would be made to preserve Turkish culture through the establishment of a special administration.

In accordance with the provisions of the Ankara Agreement, a special form of administration was established in Hatay in 1922 and remained in force until France granted independence to Syria. On 9 September 1936, an agreement was signed between France and Syria to lift the French mandate in Syria. The fact that this agreement did not contain any provisions regarding Hatay seriously disturbed both the Turkish people living in Hatay and the Republic of Turkey. This was because, with the agreement, France withdrew from Syria while leaving all its authority over Hatay to Syria. On 9 October 1936, Turkey protested this situation in a note to France, requesting that independence be granted to Syria. France, in its response, stated that granting independence to Hatay would mean dividing Syria and rejected Turkey's request.

After the exchange of notes between the two states, Turkey brought the issue to the League of Nations at France's suggestion. From this point onwards, significant public opinion formed in Turkey regarding the urgent need to resolve the Hatay issue, and everyone from all levels of government to the general public began to take a keen interest in the matter. While diplomatic negotiations between France and Turkey continued, the demonstrations held by the Turkish people of Hatay with great enthusiasm and excitement led to bloody clashes. In response to the events,

Atatürk traveled to Konya in January 1937 and then to Ulukışla to convey a warning message to France, engaging in various contacts and holding discussions with the local population. However, despite this agreement, the issue remained unresolved. Disagreements between Turkey and France over the electoral system, France's obstruction of the implementation of the constitution, and the French colonial administration in Hatay inciting the Arabs against the Turkish community were among the reasons for the renewed tension between Turkey and France.

Despite the passage of a year, no concrete steps had been taken, and in light of the ongoing tensions, Atatürk, despite his poor health and the objections of his doctors, traveled to Mersin and Adana between 20 and 24 May 1938 to inspect the artillery and infantry units, thereby demonstrating Turkey's determination in this regard. In the days that followed, the murder of a Turk in the Christian quarter of Hatay caused an uproar among Turks, and an attempt to attack the Christian quarter was barely suppressed. Turkey protested the events to France and the League of Nations, and this was postponed, and the League of Nations Commission was about to leave Hatay. As a continuation of all this, at the end of June, Turkey deployed a military force of 30,000 people to the border.

Turkey's tough and determined approach and the increasingly tense political situation in Europe, which brought the world to the brink of a new world war, led France to adopt a more moderate and conciliatory stance towards Turkey in the Hatay issue. The developments that followed this process largely proceeded in line with Turkey's wishes and expectations, favoring Turkey.