Though it continued to regard itself as the Roman Empire, Byzantium was Greek in language and culture, and it gradually shed the remaining vestiges of the old Roman imperium. The reign of the Emperor Heraclius (610–641) marked the decisive break with the Roman past. He not only restructured the imperial administration and army but replaced Latin with Greek as the official language of the state. From that point forward the Byzantine Empire was a Greek polity.

The question of what flags may have been used by Byzantium is partly a matter of conjecture. The "Imperial Standard After 395 AD" is a reconstruction based on vague ancient sources; it displays the chrismon, a symbol for Christ. The golden yellow double-headed eagle flag, however, is known to have been used by the emperors of the last Byzantine dynasty, the Palaiologos. It was adopted by Michael VIII Palaiologos after his recapture of the Byzantine capital, Constantinople, from the Crusaders in 1261. The eagle with its two heads symbolized the empire's sovereignty over European and Asian lands. The fourteenth-century Book of All Kingdoms ascribes a number of different flags to Byzantium, mostly with a cross between four Greek letters Β: the initial letters of the Paleologues dynasty's motto, Βασιλευς Βασιλεων Βασιλευων Βασιλευσιν ("King of Kings, Ruling over Kings"). The Ottoman conquest of 1453-56 extinguished the Byzantine Empire, but the double-headed eagle was adopted by the Russian tsars, who regarded themselves as the heirs of the Byzantine emperors.

Ottoman Merchant Ensigns for Greek Vessels

National Flag  •  1822-1978
 

A wide variety of war flags and banners was used by Greek armies and leaders during the War of Independence. One of the most common was a white flag charged with a light blue cross, originally the banner of the Kolokotronis family, whose members were prominent in the independence movement. A white flag with a horizontal blue stripe, dating from 1821, was flown by Greek naval squadrons. The personal standard of General Dimitris Plapoutas was a white flag with a blue cross and the Monogram I X N K, standing for "Jesus Christ Victorious." The standard or command flag of Admiral Andreas Miaoulis, the commander of the Greek fleet from 1822 until his death in 1836, was white with a broad yellow cross and a canton reminiscent of the Russian naval jack. The date 1821 referred to the year of the outbreak of the independence war. The motto, Freedom or Death, was the war cry of the independence forces.

In 1822, the Greek National Assembly proclaimed the country's independence from the Ottoman Empire. At this time the Assembly declared blue and white to be the Greek national colors and adopted three national flags: blue with a white cross for use on land, nine stripes of blue and white with a blue canton bearing a white cross for use by warships and blue with a white canton bearing a blue cross for merchant ships. Blue was said to represent the sky and white the purity of the Greek cause. The cross was a symbol of Orthodox Christianity and the nine stripes of the war ensign represented the nine syllables of the war cry of the independence struggle: Freedom or Death." Greece's independence was formalized by the Treaty of London (1830) and in 1832 the country became a kingdom, Bavaria's Wittelsbach dynasty supplying a crowned head in the person of King Otto I. The 1822 flags were confirmed except that the merchant ensign was dropped; merchant ships thereafter flew the cross-and-stripes flag. Also, a new war ensign was introduced: the cross-and-stripes flag with the crowned arms of Wittelsbach at the intersection of the cross. The war flag for use on land was the plain cross flag with the crowned arms added. Otto I followed a pro-Russian policy that did not suit British interests and he was forced off the throne in 1862. The new King, George I, was a Danish prince of the Oldenburg dynasty. His accession did not affect Greek flags, except that the war flag and war ensign now displayed a royal crown only. Up to 1975, the plain cross flag was the Greek national flag on land inside the country, while the cross-and-stripes flag was used along the coast, at sea, and overseas.