Sen. John F. Kerry accused President Bush on Thursday of undermining generations of U.S. leadership and pledged that as president he would restore the United States' alliances and aggressively confront potential terrorist threats.

Kerry, who said Bush's policies have made the United States less safe, sought to balance a bellicose tone toward al Qaeda and other terrorist organizations with overtures toward allies and global institutions such as the United Nations. "As commander in chief, I will bring the full force of our nation's power to bear on finding and crushing [terrorist] networks," the Democratic presidential candidate said in a speech here. "We will use every resource of our power to destroy."

Speaking to graduate students and war veterans, including several of his Vietnam War crewmates, Kerry turned the words of Republican Theodore Roosevelt, a Bush hero, into an indictment of what he called Bush's "unilateral" foreign policy. "Time and time again, this administration has violated the fundamental tenet of Roosevelt's approach, as he described it -- 'If a man continually blusters, if he lacks civility, a big stick will not save him from trouble,' " Kerry said.

Kerry offered no new policy proposals, but provided a more detailed explanation of his worldview and foreign policy priorities. He criticized Bush for failing to kill or capture Osama bin Laden during the battle of Tora Bora in Afghanistan, for taking a "kid glove approach" to hunting down terrorist money and for coddling Saudi Arabia. "To put it simply, we will not do business as usual with Saudi Arabia," Kerry said. "They must take concrete steps to stop their clerics from fueling the fires of Islamic extremism."

Kerry dedicated only a few minutes specifically to Iraq, where he said the United States "is in deep trouble." He said the gravest threat to U.S. security comes from "lawless states and terrorists" armed with weapons of mass destruction. He said his strategy would focus on preventing the acquisition of such weapons.

Thursday's speech marked the beginning of an 11-day period during which Kerry will outline his national security policies and highlight his military service in Vietnam and foreign policy experience in the Senate. He will mark the opening of the new National World War II Memorial in Washington, Memorial Day and the 60th anniversary of D-Day with speeches and events about terrorism, modernizing the military and strengthening national defense. In his speech here, Kerry mentioned four "imperatives" of a new national security policy: rebuilding alliances; modernizing the armed forces to deal with new dangers; deploying diplomacy, intelligence, economic power and American values to help overcome threats; and freeing the United States from its "dangerous dependence on Mideast oil."

Kerry, a 20-year veteran of the Senate Foreign Relations Committee, said he would elevate the role of global organizations such as the United Nations and NATO in carrying out U.S. policy overseas.

"There was a time not so long ago when the might of our alliances was a driving force in the survival and the success of freedom -- in two world wars, in the long years of the Cold War, then from the Gulf War to Bosnia, to Kosovo," he said. "We extended a hand, not a fist."

During the 2000 campaign, Bush promised to build stronger international alliances, but, as president, he has alienated some traditional allies with such moves as rejecting a global warming treaty and pursuing the war in Iraq. Under pressure from critics, Bush recently intensified efforts to provide the United Nations and NATO more prominent and powerful roles in Iraq, a move Kerry has advocated since 2002. This convergence of views on has blurred the distinctions between the two candidates on Iraq.

As Thursday's speech showed, Kerry is positing his long history of respect for, and cooperation with, global institutions and foreign leaders as presenting the sharpest contrast between his worldview and military strategy and Bush's. He frequently uses tough rhetoric as a way to show that Democrats are as committed to national defense as Republicans.

Since the Sept. 11, 2001, attacks, Bush, many Republicans and some Democrats have said the war on terrorism sometimes requires the United States to act quickly and decisively without waiting for the blessing of the United Nations or other nations. Kerry, in essence, agrees, but he argues that a humbler and more solicitous approach would win greater support for U.S. military operations around the world and lessen the cost in lives and money.

Bush's doctrine of preemptive military action is the perfect example, according to Kerry's foreign policy advisers. Bush has asserted the United States' right to preemptively strike a nation it deems an imminent threat to U.S. security. Kerry would not rule out preemptive strikes, nor have past presidents, but he would de-emphasize this option in stated U.S. policy. Samuel R. "Sandy" Berger, a top Kerry adviser, said this represents a "profound difference" between the two candidates.

If an attack "appears imminent, as commander in chief, I will do whatever is necessary to safeguard the country," Kerry said. And he warned al Qaeda against trying to influence the presidential election.

Still, it is unclear how Kerry's multilateralism would administer military force. In a briefing before the speech, Kerry's foreign policy advisers said it is uncertain whether the senator from Massachusetts would have waged war with Iraq if he were president.

Kerry voted for the congressional resolution authorizing military action to depose Saddam Hussein, but he cautioned that all diplomatic options should be exhausted before striking Iraq. In 1991, he voted against the war with Iraq. Kerry said he was not opposed to the military operation 13 years ago but wanted to force the use of tougher diplomatic tactics first. He fought in Vietnam but opposed the war afterward. Kerry supported the U.S. military operations in Bosnia, Kosovo and Somalia, each time pushing for a broad, multilateral approach.

He called the situation in Iraq "grim" and encouraged Bush to use his upcoming trip to Europe and the Group of Eight Summit in Georgia to enlist NATO allies. He reiterated his plan for appointing a new International High Commander to help organize elections and the writing of a constitution in Iraq.

Kerry did not focus on the similarities between his Iraq policies and Bush's. Both oppose withdrawing military forces and putting U.S. troops under international leadership, and both support increasing the number of U.S. troops if needed to stabilize the country.

The Bush campaign said Kerry offered nothing new and contradicted his earlier positions. "For John Kerry, the war in Iraq and the overall war on terror are a political game of Twister," Bush campaign manager Ken Mehlman said.

Kerry's approach toward Iraq and terrorism often contrasts with the much stronger criticism of Bush's foreign policies expressed by the growing ranks of antiwar Democrats and others. Recent polls show that a majority of Democrats want to pull out of Iraq, while former vice president Al Gore and others are calling for resignations in the Bush administration.

Staff writer William Branigin in Washington contributed to this report.