National flags are not much older than the idea of a nation, and the two grew up together. Tracing how flags changed is a way of tracing how political power changed: who claimed it, from whom, and what they wanted the claim to look like.
Origins
Flags go back to antiquity, though not in the national sense. China, India and Egypt used them as military ensigns or in ceremonies. Those early flags were simple, usually a geometric pattern or a symbol standing for a tribe or a military unit.
In medieval Europe flags came to represent feudal lords and kings. The banners carried heraldic symbols recording lineage and conquest, and the designs grew more complex as heraldry did. It was during this period that a flag started to stand for a sovereign state rather than for the individual who happened to be ruling it.
The first national flags
The national flag in the modern sense emerged with the nation-state itself, in the late medieval and early modern periods. One of the earliest is Denmark's Dannebrog, which legend says fell out of the sky during a battle in 1219. Divine right and national destiny were common themes in early flag symbolism, and that story is a compact version of both.
The 17th and 18th centuries pushed things along. The Age of Exploration and the founding of overseas colonies made it necessary to identify a ship's nationality at sea, which gave maritime flags a practical urgency. The Union Jack came out of this period, combining the crosses of St George, St Andrew and later St Patrick as England, Scotland and Ireland came under a single sovereign.
Revolutions and the flags they produced
The 18th and 19th centuries were the period that changed flags most. The American Revolution produced the Stars and Stripes, with a new star added for each state joining the union. Using a flag to encode a federal structure was a new idea, and it became a template for a great many later designs.
The French Revolution supplied the other template. The blue, white and red tricolor came to stand for liberty, equality and fraternity, and it was copied across Europe and Latin America by nations fighting free of colonial powers or monarchies.
The 20th century
The 20th century created more new countries than any before it, mostly after the World Wars and during the decolonization of Africa and Asia. Each one needed a flag, and each flag had to state an identity, an independence and a governing philosophy at once. Many African countries adopted the pan-African colors of green, yellow and red, originally used by Ethiopia, as a statement of African independence and unity.
By the end of the century certain design elements had become internationally legible. Stripes, stars and particular color combinations such as red, white and blue carried associations with freedom, peace and unity almost regardless of which country was flying them.
Where things stand
Flags continue to change, usually when politics does. South Africa's 1994 redesign marked the country's new democratic era after apartheid, blending colors associated with both the African National Congress and the old Boer republics.
They also work as instruments of foreign policy. Flags are raised at diplomatic events, at global sporting competitions and on space missions, where the message is cooperation rather than sovereignty. It is the same piece of cloth doing a different job.