No war has had more publicity than World War II. Ernie Pyle wrote War is Hell during the war. Newsreels were shown at every Saturday matinee throughout the U.S. Movie after movie was made on the subject of the European theatre, and some on the war in the Phillipines. Disillusioned poets and authors wrote of the war and the bomb. World War II was of such great magnitude that it was called "a war to end all wars." To this day, there are works written of this era: biographies of Dwight D. Eisenhower, Omar Bradley, George S. Patton; Tom Brokaw's book, The Greatest Generation, and so on. Fortitude and patriotism were prominent virtues in the soldiers of this time.
In contrast with the image the United States developed within its own identity and in the eyes of the rest of the world during WWI and WWII, the Vietnam War created serious questions within the national psyche. Americans are not accustomed to not achieving unqualified victory when engaged in foreign military actions. Returning vets from Vietnam were not given the recognition they deserved because many citizens at that time opposed the war and generalized that opposition to include anyone who had been involved with it in any way. As the US military hierarchy reevaluated itself, its policies and procedures, and its capabilities in the aftermath of Vietnam, the first doubts about the ability of the US to fulfill the role of "peacekeepers of the world" began to be seen.
Now, as the US finds itself involved in military actions in Afghanistan, Pakistan, Iraq, and other countries, I have to wonder how much we have learned from the past. We are still quick to become involved when an ally needs support, but have again become entangled in ancient conflicts and contemporary power struggles that we don't fully understand and that are going to be very costly, in monetary and human terms, to escape. I love that my country wants to help all people everywhere achieve a better life situation, but I fear we need to recognize that some struggles may be beyond our "fixing" through military intervention.
Many history classes and text books spend quite a bit of time studying WWI and WWII, especially WWII, because these wars brought to the fore the international military and economic dominance of the United States. Our national identity was tied to the spread of democracy and the protection of democratic change in countries around the world. We became known as the peacekeepers of the world.
Notable historical events, such as the signing of the Declaration of Independence and other aspects of the American Revolution, are a major focus of historical emphasis. Patriotism and heroism are two traits often associated with such events.
There are so many events that are emphasized by different teachers and/or different textbooks. Each says something different about our national character.
One example is the westward expansion of the US. This is emphasized in many texts. It illustrates the character trait of tenacity and individuality. We tend to emphasize this phase of our history because it tends to portray us as a nation of tough individuals who are not afraid to go out and make new lives for ourselves out of whatever opportunities we can find.
Your question is quite broad; but the events which seem to be emphasized more than any other are the Revolutionary War and the Civil War. The Revolutionary War demonstrated American's willingness to fight to protect the rights which they believed had been denied them and also marks the beginning of a feeling of American Nationalism.
The Civil War determined, in Lincoln's words
if any nation so conceive and so dedicated can long endure.
The primary issue was the power of the federal government versus the power of the states, although this issue was overshadowed by the slavery debate. It does demonstrate by its end the unity of the American people. Prior to the Civil War the term "United States" was considered plural; after the war, the term was considered singular,