I can't think of another country where the flag plays so central a role in history as it does in the United States. Above are 2 historical events that make conspicuous use of the flag. They are both about the central question of American history; just who exactly is included in the opening 3 words of the Constitution, "We the People..." The bottom photo is a Ku Klux Klan parade in Washington DC in the 1920s held at the height of their power. The flag to them was the emblem of America as a white Christian republic. At the top is the Selma to Montgomery March of 1964, also making conspicuous use of the American flag. For those marchers, the flag meant a legitimate, and long frustrated, claim to full citizenship in the country of their birth.
As with any symbol, the American flag has many levels of meaning, and means very different, and sometimes contradictory things, to different people. This may help explain why the flag has a kind of sacramental mystique about it for a lot of Americans.