The Balkans soon were convulsed in a major regional war, from which Greece emerged victorious and with its territory substantially enlarged. At the heart of the Balkan Wars were three issues: the disposition of Macedonia, the problem of Crete, and liberation of the countries still under Ottoman control, especially Albania.

Some Macedonians wanted full unification with Greece, others wanted a separate Macedonian state, and still others wanted Macedonia to be included in a Serbian or Albanian or Bulgarian state. This issue was appallingly divisive, and the choice often was literally a matter of life or death. Guerrilla fighters and propagandists entered Macedonia from Greece and all the other countries of the region. Athens actively supported the irredentist movement in Macedonia with money, materials, and about 2,000 troops. Thessaloniki became more of a Greek city as non-Greek merchants suffered boycotts and left. Greece's lack of access to this key port heightened tension with the Slavic neighbors.

Under these circumstances, all the Great Powers became more involved in the Macedonian problem in the first decade of the twentieth century. Britain pressured Greece to curb guerrilla activities. When the Young Turks took over the government of the Ottoman Empire with a reformist agenda in 1908, a short period of cordial negotiations with the Greeks was chilled by reversion to nationalist, authoritarian rule in Constantinople. New Ottoman intransigence over Crete and Macedonia combined with Venizelos's demand for complete reunification to raise the prospect of war in 1910.

Nationalism in the Balkans was the final element of the war that erupted in 1912. Early that year, a mutual defense pact between Serbia and Bulgaria divided northern Macedonia between those two countries. In response Athens signed bilateral pacts with both neighbors. Essentially, the three Balkan powers thus agreed to cooperate militarily against the Porte, but they did not agree on the vital question of how to distribute territory surrendered by the Ottoman Empire.

The Balkan powers initiated the First Balkan War by marshaling over 1 million troops and then declaring war on the Turks in October 1912. Venizelos's military modernization paid rich dividends. Within a matter of weeks, the Greek army took Thessaloniki and besieged Ioannina to the west. The armies of all three allies fought mainly to gain a favorable position in a postwar settlement. In the May 1913 Treaty of London, the Ottoman Empire ceded all its European possessions to the Balkan allies, with the exception of Thrace and Albania, the latter of which became independent.

Because the Treaty of London made no division of territory among the allies, and because Greece and Serbia had divided Macedonian territory between themselves in a bilateral agreement, Bulgaria attacked both, initiating the Second Balkan War. Greece and Serbia won victories that ensured major territorial gains at the Treaty of Bucharest in August 1913.

The addition of southern Epirus, Macedonia, Crete, and some of the Aegean Islands expanded Greece by 68 percent, including some of the richest agricultural land on the peninsula, and the population nearly doubled. The major Greek cities of Ioannina and Thessaloniki were reclaimed. Although more than 3 million Greeks remained in Ottoman territory, the Balkan Wars had brought the Megali Idea closer to realization than ever before. When King Constantine was crowned following the assassination of King George in Thessaloniki in March 1913, national morale had reached a high point.