"We will use force when necessary in the defense of freedom," the president said, calling Britain "our closest friend in the world."
During a 3 & 1/2-day state visit here, Mr. 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," Mr. Bush said, mindful of the bitter opposition among many in Britain and across Europe to the U.S.- and British-led war in Iraq.
But, he added: "Those in authority are not judged only by good motivations. That duty sometimes requires the violent restraint of violent men."
With as many as 100,000 people preparing to march through London Thursday to protest the Iraq war and occupation, Mr. 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.
"The failure of democracy in Iraq would throw its people back into misery and turn that country over to terrorists who wish to destroy us," he said. "Yet democracy will succeed in Iraq because our will is firm, our word is good and the Iraqi people will not surrender their freedom."
The president cast the decision to invade Iraq as a choice between action or inaction in the face of an imminent threat, and subtly contrasted his choice with Europe's past appeasement of dictators.
Mr. Bush said the last American president to make a state visit to Britain, Woodrow Wilson, was fueled by optimism that the League of Nations would prevent future wars. "Munich, Auschwitz and the Blitz" soon followed, Mr. Bush said, because "the League of Nations, lacking both credibility and will, collapsed at the first challenge of the dictators."
"Idealism requires common purpose and national strength, moral courage and patience for difficult tasks," he said. "Now our generation has need for these qualities."
Mr. Bush reiterated the defense of force that he has mounted consistently. He hailed the departure of Saddam Hussein's dictatorship, warned of the danger of terror groups getting hold of weapons of mass destruction, and said democracy in the Middle East would help improve lives and defeat terrorism.
He also called for steps to block Iran from developing nuclear weapons and steps to resolve the Arab-Israeli conflict. Anti-Bush protesters are planning a few small rallies Tuesday, including one outside the heavily guarded U.S. Embassy, in support of al Qaeda and Taliban terrorist suspects detained without trial in Guantanamo Bay, Cuba.
The real test of anti-Bush sentiment comes Thursday. The Stop the War Coalition expects 100,000 to march past Parliament and Blair's nearby Downing Street office.
Fewer than 100 anti-war protesters were standing outside Buckingham Palace on Tuesday night as the Bush entourage arrived in two helicopters. The president and wife Laura landed deep inside the walled-off estate in central London, far beyond earshot of a few dozen stalwarts shouting "Bush go home!" and other invectives.
"I think he (Bush) is a disgrace," said one demonstrator. "I think he is a disgrace to the American people and these demonstrations are in solidarity with the American people. We are not anti-American - we are anti-Bush."
Also Wednesday, Mr. Bush was to meet with relatives of British victims of the Sept. 11 attacks and British veterans of the war in Iraq. Britain has sent more troops to Iraq than any country aside from America, about 9,000, and the British have lost more than any other American ally — 52 deaths since the start of the war.
Later on the Bush schedule: a banquet at the palace with the queen, Blair and numerous dignitaries.
Mr. Bush's visit to Britain dominated broadsheet newspapers in the country Wednesday morning. Headlines concentrated on the unprecedented level of security surrounding the visit, with The Times of London's front page leading with "President strolls into Fortress Britain."
A security cordon of thousands of police officers and hundreds of American Secret Servicemen was unable to prevent an embarrassing security snafu at the palace.
A reporter for London's Daily Mirror says he had no trouble using a phony reference to get a job as a palace servant - and had he wanted to kill the Queen or Mr. Bush, it would have been easy to do.