A national flag is often taken as the essential symbol of the country concerned, as in the US Pledge of Allegiance.
In the US, 14 June, the anniversary of the adoption of the Stars and Stripes in 1777, is known as Flag Day.
flag of convenience a flag of a country under which a ship is registered in order to avoid financial charges or restrictive regulations in the owner's country.
fly the flag (of a ship) to be registered to a particular country and sail under its flag; in figurative use, to represent or demonstrate support for one's country, political party, or organization, especially when one is abroad.
put the flags out celebrate publicly. Evelyn Waugh's novel Put Out More Flags (1942) had an epigraph from the Chinese, part of which reads, ‘a drunk military man should order gallons and put out more flags in order to increase his military splendour.’
show the flag make a gesture of support for or solidarity with one's country, political party, or organization, especially when one is abroad or among outsiders. Used literally of a naval vessel making an official visit to a foreign port, especially as a show of strength.
wrap oneself in the flag make an excessive show of patriotism, especially for political ends (chiefly in North American usage).
see also trade follows the flag.