President Obama on Wednesday called for a new chapter in the long history between the United States and Britain, saying that global economic, military and climate challenges require “remaking ourselves to meet the demands of a new era.”

Even as he spoke of how much the world has changed since the allies emerged victorious from World War II, Obama told a joint session of Parliament that U.S.-British cooperation — in military, diplomatic, economic and humanitarian efforts — remains essential to ensuring global security.

“As new threats spread across borders and oceans, we must dismantle terrorist networks and stop the spread of nuclear weapons, confront climate change and combat famine and disease,” Obama said. “And as a revolution races through the streets of the Middle East and North Africa, the entire world has a stake in the aspirations of a generation that longs to determine its own destiny.”

Obama said that, despite the need for both countries to reduce public debt that could “sap the strength and vitality of our economies,” the United States and Britain must continue to remain engaged in a world in which Brazil, India, China and other developing countries are becoming important forces.

“It’s become fashionable in some quarters to question whether the rise of these nations will accompany the decline of American and European influence around the world,” Obama said. “That argument is wrong. The time for our leadership is now.”

Although he did not make new policy declarations or articulate a different relationship between the United States and its European allies, Obama celebrated a partnership that, in the past decade, has waged war in three Muslim nations and suffered through the global economic downturn.

His challenge was to argue that much of the sacrifice, in lives and money, has made a difference in creating a more stable, safer world — and that the partnership must continue its missions in Afghanistan and Libya, despite the strain on national treasuries. He said the countries “have arrived at a pivotal moment once more.”

Obama delivered his address in the historic Westminster Hall, becoming the first U.S. president to speak there.

The gray-stone Gothic building, with a high vaulted ceiling of wooden beams, dates to 1099 and served as the banquet hall for the coronation of many English kings, including Richard I. It is where Winston Churchill’s body lay in state.

In introducing Obama, the speaker of the House of Commons, John Bercow, said, “Few places reach so far into the heart of our nation,” referring to the hall.

Obama spoke at the front of the long hall, a floor-to-ceiling stained-glass window his backdrop. More than 1,000 lawmakers, government officials and other invited dignitaries gathered to listen.

In keeping with the solemnity of the setting, they showed more reserve than the U.S. Congress does during similar addresses and broke into applause only when Obama cited his family story, which he told as a parable of how the values and diversity of the British and American populations are each country’s strengths.

“The example of our nations says that it’s possible for people to be united by their ideals instead of divided by their differences,” Obama said. “That it’s possible for the sons and daughters of former colonies to sit here as members of this great Parliament, and for the grandson of a Kenyan who served as a cook in the British army to stand before you as president of the United States.”

The audience also gave Obama a standing ovation when he concluded the 34-minute speech, which served as the most substantive element of his two-day state visit to London, where he and the first lady, Michelle, have dined with royalty and barbecued with military veterans from both nations.

Obama leaves Thursday for Deauville, France, and the Group of Eight summit, where such issues as the Libya military operation, the dormant Israeli-Palestinian peace process and the Arab uprisings will be on the agenda.

In encouraging democratic transition in the Arab world, Obama said, “the West must overcome suspicion and mistrust among many in the Middle East and North Africa — a mistrust that is rooted in a difficult past.”

“For years, we’ve faced charges of hypocrisy from those who do not enjoy the freedoms that they hear us espouse,” he said. “And so, to them, we must squarely acknowledge that, yes, we have enduring interests in the region — to fight terror, sometimes with partners who may not be perfect; to protect against disruptions of the world’s energy supply.”

The comment appeared to refer to the administration’s support for several oil-rich autocracies in the region, including the royal family of Saudi Arabia.

Before the speech, Obama met with Prime Minister David Cameron, discussing many of the security issues that are also likely to arise at the G-8 summit.

During an open-air news conference, the leaders emphasized their shared commitment to toppling Libyan leader Moammar Gaddafi, saying that fulfilling the U.N. resolution to protect Libya’s civilians will be difficult with Gaddafi still in power. Obama said forcing Gaddafi out will be “a slow, steady process.”

But Obama and Cameron appeared to differ on whether Palestinians should pursue a U.N. resolution this fall that would recognize an independent Palestinian state alongside Israel.

Echoing his speech from last week, Obama said a Palestinian state would be created not by the United Nations but through a negotiated deal with Israel.

The position pleased the Israelis, who fear the resolution could increase pressure on it to end its occupation of territories captured in the 1967 Arab-Israeli war.

But Cameron suggested that the resolution could encourage both parties to return to direct negotiations, which Obama inaugurated last year in Washington, only to see them collapse within weeks.

“We want to discuss this within the European Union and try and maximize the leverage and pressure that the European Union can bring, frankly, on both sides to get this vital process moving,” Cameron said.

Regarding Afghanistan, the two leaders said they would place new emphasis on talks with the Taliban, a process of reconciliation that Obama said must be “Afghan-led.”

The two men spoke easily at side-by-side lecterns and complimented each other frequently.

They also drew an ideological contrast with the partnership shared by their predecessors, George W. Bush and Tony Blair, who jointly led the 2003 invasion of Iraq with the hopes of bringing democracy to the heart of the Arab world.

Cameron said, “Every relationship between a president and a prime minister is different.” But, he added, “we have to learn the lessons of history, about how best we promote the values that we share.”

“And that means, yes, going with the grain of other cultures,” he said. “It means, yes, having a patient understanding that building democracy takes time and you have to work on the building blocks of democracy and not believe this all can be done in an instant.”