US President Barack Obama has told America it must make "hard choices" to reduce the cost of health care and the size of its financial deficit.
But the president said every citizen deserved a basic measure of security and dignity, and he held up Medicare, Medicaid and Social Security as commitments that strengthen the country.
Speaking in his inaugural address at the US Capitol, Mr Obama said he rejected the belief that the country must choose between caring for the generation that built the country - or investing in the generation that will build the future. Struggles with Republicans over reducing the deficit loom for him in his second term.
Mr Obama promised to respond to the "threat of climate change" and lead the way on renewable energy sources. And he declared that a decade of war was ending, the nation's economy was recovering and "America's possibilities are limitless".
"My fellow Americans, we are made for this moment, and we will seize it, so long as we seize it together," he said, moments after taking the oath of office. Mr Obama's address touched on the broad gifts that bring the country together, and pointed to the work ahead, "the realities of our time."
He was officially sworn in on Sunday, as required by law, but the glitter of Inauguration Day - the parade down Pennsylvania Avenue toward the White House, the night of balls, the ceremonial beginning of a new four-year presidential term - still enlivened staid Washington.
The celebration was pushed to Monday because January 20 fell on a Sunday this year. That placed the grand ceremony on the US holiday marking the birthday of revered civil rights leader Martin Luther King.
Mr Obama referred to the Stonewall gay-rights riots in his address, classing them as a civil rights watershed along with key moments in the struggles for blacks and women. The president said that the truth that all are created equal guides us today "just as it guided our forebears through Seneca Falls, and Selma, and Stonewall."
The Stonewall riots happened in New York in 1969 when patrons at a gay bar reacted to police harassment, and the events helped found the modern gay-rights movement.
Mr Obama, who has become increasingly outspoken in favour of gay rights and same-sex marriage, also said the nation's journey is not complete "until our gay brothers and sisters are treated like anyone else under the law."