"In some cases, the measured use of force is all that protects us from a chaotic world ruled by force," Bush told academics gathered at Whitehall Palace.

During a 3 1/2-day state visit here, Bush was seeking to puncture what he views as misconceptions on this side of the Atlantic about America's use of force in Iraq.

"There are principled objections to the use of force in every generation and I credit the motives behind these views," Bush said, mindful of the bitter opposition among many in Britain and across Europe to the US-and British-led war in Iraq.

But, he added: "Those in authority, however, are not judged only by good motivations. The people have given us the duty to defend them and that duty sometimes requires the violent restraint of violent men."

He invoked the Sept. 11, 2001, terrorist attacks in America to warn against inaction.

"The hope that danger has passed is comforting, is understanding, and it is false," he said. "These terrorists target the innocent and they kill by the thousands. And they would, if they gain the weapons they seek, kill by the millions and not be finished. ... The evil is in plain sight. The danger only increases with denial."

Earlier, Queen Elizabeth II and Prince Philip gave a royal salute to the American leader, greeting Bush at Buckingham Palace.

As ceremonial cannon blasts from a 41-gun salute shook the palace, Bush and his wife, Laura, moved down a receiving line with the queen and prince, greeting Prime Minister Tony Blair, Foreign Secretary Jack Straw and a phalanx of military officers in formal dress. Bush and the prince then inspected a column of Coldstream Guards, with their trademark gray coats and tall, furry black hats.

Buckingham Palace, the queen's London residence, also was a focal point for demonstrators bitterly opposed to the Iraq war. They gathered behind metal barriers Wednesday, watched by large numbers of yellow-jacketed police officers.

But though the light crowd of protesters was kept several dozen yards from the gates, their chants could be heard inside the grounds as the president greeted dignitaries.

In his speech, Bush subtly invoked Europe's history of appeasement of dictators, reminding his audience of the critical work the Allies did to set postwar Germany on the path to democracy -- and thanking the British for their help in setting Iraq on a different course now.

"Let us never forget how Europe's unity was achieved," he said. "Together our nations are standing and sacrificing ... in a distant land at this very hour."

With as many as 100,000 people preparing to march through London Thursday to protest the Iraq war and occupation, Bush acknowledged "good-faith disagreements in your country and mine over the course and timing of military action in Iraq."

But he warned against breaking the coalition's commitment to see Iraq through to a stable democracy.

Blair, who has faced strong criticism from the British public for remaining America's staunchest ally in the Iraq war, also defended Britain's close relationship with the United States and the coalition's handling of the situation in Iraq.

In a friendly jab at France, Bush noted how a 14-point plan for peace that President Woodrow Wilson took to Britain in 1918 was met with skepticism by the prime minister of France, who complained then that even God himself "only had 10 commandments."

Thursday, Bush was to sit down with family members of British soldiers killed in Iraq.