HANOI, Vietnam -- President Bush, opening a visit today to the wartime capital of this once-divided country, stirred inevitable comparisons between the unpopular war raging in Iraq and the divisive war fought and lost in Vietnam more than three decades ago.

His itinerary promised some interesting moments. Before attending a state dinner this evening, Bush was to drop by the headquarters of the Communist Party to talk with its general secretary.

Bush was the fourth U.S. president to visit Vietnam, where communist forces prevailed over the United States and a Washington-backed regime in Saigon in a conflict that claimed the lives of more than 58,000 Americans. President Clinton visited Vietnam in 2000; Lyndon Johnson and Richard Nixon made wartime visits.

Bush and his aides have pushed back against comparisons of the war here and the Iraq war, now in its fourth year. Like Vietnam, the United States faces a determined insurgency in Iraq; both wars have demonstrated the limits of U.S. power.

"Historic parallels of that kind are, I think, not very helpful, and I don't think they happen to be right," Secretary of State Condoleezza Rice told reporters earlier. "This is a different set of circumstances with different stakes for the United States in a different kind of war."

Bush flew here from Singapore after warning a nuclear-armed North Korea against peddling its weapons and vowing the United States will not retreat into isolationism.

Although Republicans lost control of Congress, Bush directly challenged newly empowered Democrats who are demanding a fresh course in Iraq and fearful that free-trade agreements could cost U.S. jobs.

"We hear voices calling for us to retreat from the world and close our doors to these opportunities," the president said in a speech at the National University of Singapore. "These are the old temptations of isolationism and protectionism, and America must reject them."

Bush came to Vietnam for a summit of Asia-Pacific leaders and individual meetings with a handful of leaders -- all of them curious whether election setbacks had unsettled Bush.

Bush will draw on his powers of personal diplomacy in meetings Saturday and Sunday with Russia's Vladimir Putin, China's Hu Jintao, Japan's Shinzo Abe and South Korea's Roh Moo-hyun. All are partners with the United States in talks aimed at persuading a defiant North Korea to abandon its nuclear weapons.

While North Korea's nuclear test has been widely condemned, the 21 members of the Asia-Pacific Economic Cooperation forum appeared divided over what to say publicly. Rice went to Vietnam ahead of Bush to seek a consensus.

Bush said the transfer of nuclear weapons or material by North Korea to others would be "a grave threat to the United States, and we would hold North Korea fully accountable for the consequences of such action."

"For the sake of peace," he said, "it is vital that the nations of this region send a message to North Korea that the proliferation of nuclear technology to hostile regimes or terrorist networks will not be tolerated."

Bush's message in Asia was clear: The United States has been a power in Asia for more than six decades and isn't about to pull back now. Many nations in the region are nervous about the rise of China and how Washington will react.

Despite Bush's tough talk, he was unable to deliver a promised agreement to normalize trade with Vietnam. The accord was held up by the House, sending a bad signal across Asia about Bush's clout and the future of trade-liberalizing bills in the Democratic Congress taking power in January.

White House National Security Adviser Stephen Hadley readily admitted the administration was disappointed about the Vietnam trade bill but expressed optimism it would pass.

"In this new century," Bush said, "America will remain engaged in Asia, because our interests depend on the expansion of freedom and opportunity in this region."

He said the United States sees its role in Asia, a region with a history of colonialism, as one of "partnership, not paternalism."

"And the United States makes this pledge: Every nation that works to advance prosperity, health and opportunity for all its people will find a ready partner in the United States," Bush promised.

In Singapore, Bush met with Prime Minister Lee Hsien Loong. He also paid a courtesy call on acting President J.Y. Pillay and lauded Singapore's success at integrating its many ethnicities and religions by visiting its Asian Civilisations Museum.

Lee, who often has advised Bush on how to improve the U.S. image, particularly in the Muslim world, seemed pleased with the president's focus. "Singapore is very happy that America has a stake in the region, and is growing the stake in the region," Lee said.

Bush told the prime minister that "America's presence in the Far East is very important for our own country."

As he left for Asia, Bush had another foreign-policy priority on Capitol Hill -- a civilian nuclear pact with India. While in Singapore, Bush spoke by telephone with Indian Prime Minister Manmohan Singh about the agreement, which had been approved by the House and was overwhelmingly endorsed by the Senate on Thursday.