United Nations » President Barack Obama challenged world leaders Wednesday to shoulder more of the globe's critical burdens, promising a newly cooperative partner in America but sternly warning they can no longer castigate the U.S. as a go-it-alone bully while still demanding it cure all ills.

"Those who used to chastise America for acting alone in the world cannot now stand by and wait for America to solve the world's problems alone," said Obama in put-up-or-shut-up comments before a packed U.N. General Assembly hall. "Now is the time for all of us to take our share of responsibility for a global response to global challenges."

In his first appearance before the group, Obama promised the U.S. would reach out in "a new era of engagement based on mutual interest and mutual respect," but he also wagged a rhetorical finger at leaders who spend much of their time at international gatherings excoriating the U.S. He said "an almost reflexive anti-Americanism" that swept the globe under the administration of his predecessor, George W. Bush, is not "an excuse for collective inaction."

"Nothing is easier than blaming others for our troubles and absolving ourselves of responsibility for our choices and our actions," he said.

And yet, directly following Obama at the podium was Libyan leader Moammar Gadhafi, who railed against the U.N. Security Council, which includes the U.S., calling it a "terror council" and accusing it of treating smaller nations as "second class, despised."

U.S. presidents -- Bush included -- have come to the United Nations year after year with a wish list of action items and preaching the gospel of working together. The U.S. is rich and powerful, but cannot solve problems without help, they say, whether Democrat or Republican.

So Obama's message was not new.

But it was delivered in an unmistakably new, more humble tone.

Following a president criticized for making my-way-or-the-highway "requests" of allies, Obama didn't demand so much as he chided and cajoled. It's now an inextricably interconnected world, he said, so that each country's problems become the others'.

"In the year 2009 -- more than at any point in human history -- the interests of nations and peoples are shared," Obama said.

Following a president pilloried for arrogance, Obama talked more modestly about the United States.

To be sure, he listed American contributions. But this was no chest-thumping bragging; instead it was a more lawyerly argument aimed at convincing the jury of Obama's world peers that the U.S. has heard the complaints and, under his leadership, is addressing them. That ranges from banning torture to winding down the Iraq war, working to rid the world of nuclear weapons, aggressively pursuing Mideast peace and bringing new energy to the battle against climate change.

And he delivered the message that America will not behave as if it is better.

"No one nation can or should try to dominate another nation," Obama said. "That is the future America wants -- a future of peace and prosperity that we can only reach if we recognize that all nations have rights, but all nations have responsibilities as well."

At home, it remains to be seen whether Obama's critics on the right will see this sort of talk as giving away some of America's accepted status as the globe's lone superpower.

Many were already criticizing Obama along these lines after previous speeches meant to reach out a conciliatory hand -- such as during his inauguration or in Cairo to the Muslim world. As John Bolton, a U.S. ambassador to the U.N. under Bush, said before Obama's trip: "Why should we not expect a visible demonstration of Obamamania at the U.N.? He is giving them pretty much what they ask for."

The president's reception in the traditionally staid U.N. hall was hardly Obamamania. But he received several rounds of applause, something rarely afforded to Bush. Iranian President Mahmoud Ahmadinejad, perhaps Obama's chief foe in the room who delivered his own address later, listened intently but did not clap.

Even while offering new cooperation from Washington, Obama was blunt that others must step up or face dire consequences: "extremists sowing terror in pockets of the world, protracted conflicts that grind on and on, genocide, mass atrocities, more nations with nuclear weapons, melting ice caps and ravaged populations, persistent poverty and pandemic disease."

At the top of Obama's urgent challenges are the nuclear programs of North Korea and Iran, the first having already produced several atomic bombs, the second suspected of moving rapidly in that direction and both in defiance of repeated international demands. He said the two nations "must be held accountable" if they continue, without mentioning the tougher sanctions that are his preferred penalties.

"The world must stand together to demonstrate that international law is not an empty promise," Obama said.

The president was particularly muscular on the need to tackle global warming, declaring that America's days of dragging its feet on the issue are over. "If we continue down our current course, every member of this assembly will see irreversible changes within their borders," he said.

Seeking to build on his three-way meeting in New York on Tuesday with Israeli and Palestinian leaders, Obama urged nations aligned with either side to abandon old divides -- by speaking honestly to the Israelis about the Palestinians' legitimate claims to land and livelihood and to Palestinians and Arab nations about Israel's right to exist.

"All of us must decide whether we are serious about peace, or whether we only lend it lip service," Obama said.

He said that all leaders will be held accountable by their citizens. "They will not long tolerate those who are on the wrong side of history," he said.

And yet the problems he said require action are enormously complex and have bedeviled the world for decades. Also, when national interests collide with global priorities, leaders almost always choose the former, or pay a steep price politically. Obama himself said, "I will never apologize for" acting in America's interests.