The precipitating factor in the Greek War of Independence was the revolt of the brigand Ali Pasha, the most infamous of the local Ottoman authorities who profited from the weakening control of Constantinople over its empire. Called the Lion of Ioannina (a city in northwest Greece), Ali rebelled against Sultan Mahmud II by building a sizeable and wealthy personal fiefdom that threatened the sultan's rule in the southern Balkans. In 1820 Mahmud's decision to curb Ali ignited a civil war that provided the opportunity for the Filiki Etaireia and its leader Alexandros Ipsilantis to launch a Greek uprising.

As it developed, the revolution was pursued by various groups with a multiplicity of goals and interests throughout the region, united by a crude plan of action. Hostilities were to begin in and focus on Moldavia and Wallachia to the north, where Ipsilantis and his army of 4,500 men were located. Once these areas had been liberated, the rest would follow. Shortly after Ipsilantis crossed the Prutische (Prut) River from Russian Moldavia into Ottoman territory in March 1821, the uprising spread throughout much of the peninsula. A second front opened within weeks, when Bishop Germanos of Patras raised the flag of revolution at a monastery in the Peloponnesus. The rapid defeat of Ipsilantis in the summer of 1821 shifted the war permanently to the south.

From the beginning, the cohesiveness of the Greek revolution was limited by class differences. The chief goal of the Greek upper classes was to rid society of the Turks, the military classes sought independent enclaves for themselves in imitation of Ali Pasha, and the lower orders simply desired to escape taxation, increase their property, and move up the social scale. Diaspora Greeks also returned home with dreams of a resurrected democratic past. Keeping these competing and disparate interests together proved one of the greatest challenges of the war.

The first of the war's two phases, from March 1821 to December 1823, was largely a successful insurgency. During this period, the Greek forces were able to capture many major strongholds of the Peloponnesus and establish a strong presence in central Greece. Victories on land were coupled with successes at sea, most notably the sinking of the flagship of the Ottoman navy in November 1822. Total casualties in the first phase have been estimated at 50,000, many of whom were civilians massacred by both sides.

In spite of the rebels' early victories, political stabilization eluded them. After an initial congress in 1821, which formed a government under a new constitution, factionalism soon led to the creation of rival governments. In April 1823, the Second National Congress selected a new government, under the presidency of Petrobey Mavromihalis, which became the third body claiming legitimate rule over the Greek people. The lack of political unity, which at times degenerated into actual civil war, was to prove very costly.

The Greek War of Independence touched a chord in Western Europe. Figures such as the British romantic poet Lord Byron found a "noble cause" in the Greek struggle against the Ottoman Empire. Philhellenes, as these sympathizers came to be called, played a critical role in the war. The conflict attracted the physical, monetary, and moral support of a variety of West European idealists. The Philhellenes raised money to support the insurgents, and they focused the attention of the outside world on the conflict until the powers of Western Europe decided to intervene.