The government of George I: Prince Wilhelm of Denmark succeeded Otto as George I (1863–1913). In his favor, Great Britain renounced the protectorate over the Ionian Islands and paved the way for unification with the mother country in the Treaty of London of March 29, 1864. The new constitution, which was adopted on October 29, 1864, remained unchanged until 1911 in force. An uprising that broke out in Crete in 1866 was secretly supported by the government and openly by private individuals. In the crisis of the war between Serbia-Montenegro and Turkey of 1875/76 Prime Minister C. Trikupis to maintain neutrality against public opinion. In the Turkish-Russian war of 1877/78 Great Britain prevented the annexation of Thessaly by Greece. The Berlin Congress (1878) left the dispute over Thessaly to direct Greek-Turkish negotiations. With the mediation of the great powers, Turkey ceded Thessaly and the area of Arta to Greece on May 24, 1881 in the Convention of Constantinople. When Trikupis left politics in 1895, the Panhellenic idea took off. After a Greek expeditionary force landed on Crete in February 1897 and the intervention of the great powers was unsuccessful, Turkey declared war on Greece in April 1897. Prince Constantine’s armyin Thessaly there was a defeat, Athens had to recall its troops from Crete and on May 20th agree to an armistice. The peace treaty of December 4, 1897 provided for war compensation and border regulation in favor of Turkey.
Since 1882, the economic development was marked by the attempts of Prime Minister Trikupis to open up the country, expand the internal market, modernize and upgrade the administration. Between 1879 and 1890, Greece borrowed 630 million gold francs abroad, the majority of which, used for purchases, also remained abroad. In 1893 the debt service amounted to around 33% of the state revenue and trikupis had to declare national bankruptcy. The long-term policy of infrastructural development came to a standstill before the estimated benefits were visible as a result of the additional financial burdens caused by the international raisin sales crisis and the war of 1897 against the Ottoman Empire. The creditor countries imposed international financial control on Greece.
In December 1898, Prince George of Greece was appointed governor of Crete by the great powers; but he resigned his office in September 1906, since the agitation for the unification of the island with the motherland did not subside. In view of the country’s military weakness, the government in Athens did not dare to accept the union with Greece proclaimed by the Cretan National Assembly in October 1908. Parallel to the Young Turks Revolution (Young Turks), a military league wasformed in the Greek army in 1909, which intervened massively in domestic politics. From her became the political leader of the Cretans, E. Venizelos, called to Athens. The years of his reign (1910–15) were the heyday of modern Greek history: joining the Balkan Federation, reorganizing the armed forces, victorious participation in the Balkan wars of 1912/13.
In the First Balkan War, Greece acquired Macedonia with Thessaloniki, Epirus and the islands off the Turkish coast without the Dodecanese. These gains and the unification of Crete with Greece were no longer endangered by the Second Balkan War, only Northern Epirus remained in dispute. Between the wars, on March 18, 1913, King George I was murdered while visiting Thessaloniki. He was followed by his eldest son as Constantine I (1913-17 and 1920-22).
Greece in the First World War: At the beginning of the First World War, Constantine I, who had been married to Sophia von Hohenzollern, the sister of Emperor Wilhelm II, since 1889, declared the neutrality of his country, but this came in opposition to Prime Minister Venizelos, who entered the country Greece wished to go to war on the side of the Entente (especially Great Britain, France), since he saw such participation in the war as the best way to achieve territorial expansion of his country, especially the gain of Constantinople. After the failure of the Franco-British offensive against the Turkish positions in the area of the Dardanelles (February 1915) the king continued Venizelos as Prime Minister from, established the neutrality of his country and ruled with his devoted heads of government (including Alexander Zaïmis [* 1855, † 1936]) on. The dispute between Constantine I and Venizelos split the population into two warring camps (“national division”, Greek dichasmos). In October 1915 the Entente powers occupied Thessaloniki – at the instigation of Venizelos – in order to bring the straits under their control and to put the Greek king under pressure. Venizelos formed under British-French protectionon October 18, 1916 there was a counter-government. After a blockade of the Greek coast by the Entente and a partial occupation of northern Greece, Constantine I abdicated on June 11, 1917 in favor of his second son Alexander and left the country together with heir apparent George. Now again Prime Minister of his country (1917-20), Venizelos declared war on the Central Powers (Germany and Austria-Hungary) and their allies (Bulgaria, Ottoman Empire) and achieved recognition as a victorious power after the capitulation of the Central Powers and their allies (1918) on the part of the Entente.
From the Paris Suburb Treaties (1919/20) to the Peace of Lausanne (1923): In the Treaty of Neuilly-sur-Seine (November 27, 1919) between the Entente Powers and Bulgaria, Greece received southern Macedonia, in the Peace of Sèvres between the Entente Powers and the Ottoman Empire (August 10, 1920) Eastern Thrace (including Gallipoli) to the Çatalca line, all Aegean islands (with the exception of Rhodes) and Smyrna (occupied by Greek troops since May 1919). After Turkey, which had regained strength under the leadership of M. Kemal Ataturk, refused to ratify the Treaty of Sèvres, Venizelos sought, with the approval of the Entente, to force the acceptance of this treaty militarily and launched an offensive against the Turkish troops in Asia Minor.
In this situation, however, there was a domestic political turnaround in Greece. After the death of the young king Alexander (October 25, 1920) Venizelos suffered a defeat in the parliamentary elections that had become necessary and left the country. Due to a referendum, Constantine I returned to the throne on December 19, 1920 and, despite concerns, continued the war against Turkey. When the Greek army collapsed in the course of Turkish counterattacks in Asia Minor, he was forced by a military coup to abdicate again on September 27, 1922, this time in favor of his first son, who ascended the throne as George II.
After the armistice of Mudania (October 10, 1922), Greece again lost Eastern Thrace and its areas of Asia Minor (“Asia Minor Catastrophe”) in the Treaty of Lausanne (July 24, 1923). In addition, it had to accept 1.5 million Greeks from Asia Minor in exchange for 600,000 Turks. The Greek culture in Asia Minor, which went back to antiquity, was thus deprived of its basis.