WASHINGTON — Barack Obama became the 44th president of the United States on Tuesday afternoon, banishing forever more than 200 years of history during

WASHINGTON — Barack Obama became the 44th president of the United States on Tuesday afternoon, banishing forever more than 200 years of history during which the office passed from white hands to white hands and promising a new era of “hope over fear, unity of purpose over conflict and discord.”

Under the Constitution, Obama became president at noon ET, even though he had not formally been sworn in with the inaugural ceremonies running behind schedule. Using his full name, Barack Hussein Obama, the new president took the oath of office at 12:05 p.m. from Supreme Court Chief Justice John Roberts, whose nomination to the court he opposed as the junior Democratic senator from Illinois.

Clasping hands with his wife, Michelle, Obama smiled and waved to the crowd of as many as 2 million people who jammed the National Mall.

As the crowd chanted “Obama, Obama,” the president thanked his predecessor, George W. Bush, and said he was “humbled by the task before us, grateful for the trust you have bestowed, mindful of the sacrifices borne by our ancestors.”

Obama acknowkledged that “we are in the midst of crisis.”

“Our nation is at war, against a far-reaching network of violence and hatred,” he said. “Our economy is badly weakened, a consequence of greed and irresponsibility on the part of some, but also our collective failure to make hard choices and prepare the nation for a new age. Homes have been lost, jobs shed, businesses shuttered. Our health care is too costly, our schools fail too many, and each day brings further evidence that the ways we use energy strengthen our adversaries and threaten our planet.”

Saying ”the time has come to set aside childish things,” Obama declared: “Today, I say to you that 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.”