The Pledge of Allegiance

The United States formally adopted the Pledge of Allegiance in 1892. The large wave of immigrants filling the population disturbed some Americans, and they thought a pledge would properly indoctrinate them and prevent "negative" influences.
Common use
Just after its adoption, the Pledge of Allegiance entered the public school system and most schools began the day with the recitation. Specifically, its formal installment in school systems coincided with Columbus Day and the World's Expo in Chicago. After this, many civic organizations weaved it into tradition and routines.
First alterations
One of the first changes occurred in 1923 when the government thought new immigrants might confuse a pledge to the flag with their former countries flag. It was changed to "the flag of the United States of America." This change still remains. Franklin Roosevelt instigated another dramatic change. When recited, the original salute was the right arm extended out and up toward the flag; however, the Nazis adopted a similar salute, so the hand over the heart replaced it.
"Under God"
Still the most controversial change, and debated today, the addition of "Under God" in the Pledge began in the 1940s by religious groups, and made its way to the White House in the 1950s. Although Truman never strongly supported the change, Eisenhower did, and formalized the change during his tenure. He explained the change separated the United States from the Godless Soviet Union.
Although the Pledge of Allegiance is an institution in the United States, its use and wording has changed significantly in the last hundred years. In the future other changes may occur, molding an essential part of American culture.