Four planes were hijacked, three of them were rammed into signature American buildings, and the terrorist threat came home for the first time. This changed everything, the commentators chanted, and no one disagreed. But it wasn’t the great historical change that so many people expected, and it’s worth understanding why.

All this happened five years ago Monday, one of those days (it was a Tuesday, as unforgettable a part of the narrative as the fact that Pearl Harbor was a Sunday) that seems to define an era. That much no one will contest. Sept. 11, 2001, was unlike any day ever in American history. Serious people said it was the worst single day in American history, and maybe it was.

It was a harbinger of change. The word “security” took on an entirely new meaning, often modified by the more intimate word “personal.” Security was, more than wealth or position, the thing we craved most in that sad September five years ago, and it is, alas, the thing that has eluded us the most.

So I am not arguing that the last five years have been a period of inconsequence or that nothing has changed. Much has, and some of the worrisome change comes not from abroad but from home, where the danger is that our quest to preserve our values and our freedoms could jeopardize those very values and freedoms.

If we are to be watchmen on the walls of freedom – a phrase from the speech John F. Kennedy never delivered in Dallas on Nov. 22, 1963, another date that lives in memory and infamy – we need to watch both sides of the wall. (One of my greatest teachers, John Sloan Dickey, who was no liberal, used to say that the danger in long political struggles is that the nature of your foreign enemy can sometimes wipe off on your domestic character.) But the purpose here is to say that for all of the momentous events and changes that have occurred in the last five years, there have been periods of change more momentous than this.

Surely the period 1860-1865 etched deeper changes than did 2001-2006. In that period, Abraham Lincoln was elected, re-elected and killed; slavery caused a civil war and then was wiped from the country; 11 states left the Union and then began the very bitter process of returning to it, defeated, demoralized and demonized; a great transcontinental railroad was begun that would transform the way Americans thought about themselves and their prospects; an ambitious program began to promote land-grant institutions that themselves have made massive contributions to American agriculture, industry and scholarship.

So, too, did the period 1940-1945, when the United States slowly mobilized for conflict and then entered a two-theater world war that wiped Nazism off the face of Europe but elevated communism to a national-security threat; when the country harnessed its industrial might to outproduce the Axis and every other nation on Earth; when manufacturers invited women into the factory and changed the face of the work force forever; and when the industrial needs of the nation prompted a landmark migration of blacks from the rural South to the urban East and Midwest.

But those were periods of wartime. Ours is a period of relative peace, though wars still rage in Afghanistan and Iraq, the latter with the potential to be one of the longest American military engagements in history. So it might be useful to consider how much change occurred in another five-year period, that of 1946 to 1951.

This was the period just after World War II, and a few figures make for a compelling case. Per capita consumer expenditures rose some 30 percent in that period. Average nonfarm wages increased nearly 40 percent. The gross domestic product grew 50 percent. The number of students graduating each year from college about tripled. Steel production grew 60 percent. Automobile production more than doubled. Automobile registration grew by 50 percent. My favorite figure: Consumer expenditures for dentistry rose 14 percent.

Not that the country wore a smile in that period (though the birthrate did soar, one measure of domestic happiness, and the detergent Tide was introduced, a measure of domestic cleanliness, sometimes regarded as being close to godliness). That’s the period that included the communist revolution in China, the blockade in Berlin and the Korean War. It’s when the Soviets got the bomb, McCarthyism began, the Rosenbergs were convicted, Gen. Douglas MacArthur was fired and “1984” was published.

It was an era of great and monumental change, so much so that it is remarkable to consider a two-month period in 1951 alone, when the transistor was invented, King Abdullah I of Jordan was killed, Greece and Turkey joined NATO and, perhaps most important of all, Bobby Thompson of the New York Giants hit a ninth-inning home run off Ralph Branca of the Brooklyn Dodgers to win the National League pennant.

We lived through history on Sept. 11, 2001, and this has been an important and potentially portentous period in national and world history. Everything changed. But history reminds us of another important lesson. Everything always changes.