WASHINGTON » Some countries are founded upon ethnicity, others upon geography. Some are created from conquest or carved out by faraway rulers. But America is different: It was created from whole cloth - a story built by a people who just keep adding chapters as they go along.

It is simply what we do - and, on the occasion of a presidential inauguration, what we do best. And at noon yesterday, whatever your political affiliation, it was hard to argue that anything less than an entirely new and different chapter was at hand.

In the words of the story's newest scribe, Barack Obama, it will be - must be - a chapter about a land long considered brash and adolescent finally coming of age. “;We remain a young nation,”; the young president said. “;But in the words of Scripture, the time has come to set aside childish things.”;

This is a country established on a notion of continuous renewal and abiding belief that something wonderful - prosperity, success, justice, glory - is forever around the next bend. And there are, too, junctures in history when man and moment converge to form a pivot point when anything seems possible.

The American story has taken myriad forms and worn many coats. A shining city on a hill. We, the people. Manifest Destiny. No more auction block. Huddled masses yearning to breathe free. Brother, can you spare a dime. A date which will live in infamy. The torch has been passed. Morning in America. United we stand. Yes, we can.

Many help to define and advance the story. But in uncertain hours, it falls to the president - particularly at the starting gate of an administration - to summon history, reframe the vision and add to it in a manner relevant and inspirational to the times.

“;We encounter each other in words,”; the poet Elizabeth Alexander said in celebration of Obama's inauguration. And with his inaugural address, Barack Obama not only added to the American story but indicated at every turn that he fully understood its role in the moment - “;a moment,”; he said, “;that will define a generation.”;

Among the strands of the national tapestry that Obama recognized and teased out in his words:

» The hardworking American as groundbreaker and inspiration: “;It has been the risk-takers, the doers, the makers of things - some celebrated but more often men and women obscure in their labor - who have carried us up the long, rugged path towards prosperity and freedom.”;

» The instinct to dream big: “;There are some who question the scale of our ambitions - who suggest that our system cannot tolerate too many big plans. Their memories are short. For they have forgotten what this country has already done.”;

» The constant tension between liberty and security, particularly after 9/11: “;We reject as false the choice between our safety and our ideals.”;

» The idea of the United States as a grand experiment in self-determination: “;Let it be said by our children's children that when we were tested we refused to let this journey end.”;

» The legend of the indomitable American spirit in tough times: “;The challenges we face are real. They are serious and they are many. They will not be met easily or in a short span of time. But know this, America - they will be met.”;

» And, of course, the uneasy racial history that Obama both embodies and defies - how “;a man whose father less than 60 years ago might not have been served at a local restaurant can now stand before you to take a most sacred oath.”;

Why is it so important for a new president to understand all the chapters of this epic national story, to push it along? Because in a multiracial, multiethnic, polyglot society, only the ever-unfolding tale of applied idealism offers a common reference point and suggests a common purpose.

“;There's nothing organic about the United States. It's an invented country. And if we stop telling the American story, we're just a bunch of folks,”; says Richard Slotkin, a prominent historian of the American frontier, the place where a sizable chunk of the American story was written.

“;We're too various. You can't go ancestral,”; he says. “;The French can always go back to what it is to be French, but the Americans have to keep inventing what it is to be an American. It used to be white men conquering the wilderness. Now it has to include the story of a black man from Hawaii becoming president. You have to keep adding new chapters to the story and making the story make sense.”;

Americans' hunger for renewal can be a two-faced beast. It is a motivator, a font of creativity and drive and energy. But it also encourages short attention spans and, in its more acquisitive incarnations, fiscal irresponsibility.

That pursuit of the Next Big Thing often leaves American history misunderstood, misused, misapplied, consumed as a commodity. It's never easy to define what is genuinely “;historical”; in an age when everything from a car dealer's sale to photos of Angelina Jolie's twins is billed enthusiastically as “;historic.”;

But there was an overwhelming feeling yesterday that the launch of the Obama presidency was not only historic but historical. “;Future generations will mark this morning as the turning point for real and necessary change in our nation,”; said Sen. Dianne Feinstein, a California Democrat.

Her party affiliation may have primed her to offer such an assessment. But even the Rev. Rick Warren, a conservative evangelical pastor who gave the inaugural invocation, called yesterday “;a hinge point in history.”;

Of course, any moment in history must be evaluated not only by what led up to it, but what lies ahead. Bill Clinton, for example, has lamented that he became president at a time where he could not truly test his mettle.

The same cannot be said for Obama. He is charged with not only tackling the faltering economy and negotiating his way through the complex thicket of global affairs, but, too, with no less than defining a new post-Cold War generation ready to make its mark in a world remade.

“;This will be situated historically as one of those moments that is a window of opportunity,”; says Peter Kastor, a cultural historian at Washington University in St. Louis. “;Sometimes those windows go open, and sometimes they slam shut. But those windows are what make something a true historical moment.”;

More than three centuries ago, a story began on the American continent. It lurched forward, voraciously demanding new material, new inspirations. It created institutions, minted heroes, began to have a history. It added to itself for better and worse.

And yesterday, it arrived at a malleable moment. A man memorable because of his political acumen, his way with words and, yes, the color of his skin became a leader and placed himself squarely at the center of the American story with, it appeared, full knowledge of the past that now rests upon his shoulders.

That sense of renewal, of possibility, electrified Harold Haizlip, 73, of Los Angeles, whose great-grandmother was a slave. He grew up during the Jim Crow era but lived long enough to watch from the National Mall as Obama took office.

America, Haizlip said, stepped forward this week. “;It was what it was, but we've gone past that,”; he said. “;We are a new nation today.”;

The American story: A 239-year-old experiment in a constant state of renewal redefines itself once again - ready and hungry, as always, for the next chapter to be written so we can discover what's around the next bend.