President Bush chose the fifth anniversary of the U.S. led invasion to defend the Iraq war as the key to fighting global terrorism.

Speaking before troops, officers and defense officials at the Pentagon, Mr. Bush admitted that the costs of the war and the number of lost lives had been much higher than he had expected. But he insisted nonetheless that the invasion of Iraq that the United States began in 2003 had contributed to the safety of the world.

“Five years into this battle, there is an understandable debate over whether the war was worth fighting, whether the fight is worth winning, and whether we can win it,” he said. “The answers are clear to me. Removing Saddam Hussein from power was the right decision, and this is a fight that America can and must win.”

Considering the loss of nearly 4,000 military men and women in Iraq and the $500 billion spent on war costs, the war opponents criticized Bush’s attitude and called for a troop withdrawal. But Mr. Bush is very determined not to allow any withdrawal that could undermine the United States’ security gains in Iraq.

“War critics can no longer credibly argue that we're losing in Iraq -- so now they argue the war costs too much," Bush said. "No one would argue that this war has not come at a high cost in lives and treasure. But those costs are necessary when we consider the cost of a strategic victory for our enemies.”

“We have learned through hard experience what happens when we pull our forces back too fast. The terrorists and extremists step in,” the president argued. “They fill vacuums, establish safe havens, and use them to spread chaos and carnage.”

A few hours after the speech, Al-Qaeda chief Osama bin Laden released a video message in which he expressed his desire to fight US both in Iraq and Afghanistan.

He said that the “savage acts” of the U.S. military troops “haven't ended the war, but rather have increased our determination to cling to our right, avenge our people and expel the invaders from our country.”

The president’s speech raised the anger of anti-war activists, who protested in several places across Washington, in Miami, New York, Chicago, Los Angeles and San Francisco on Wednesday.

The war issue also separates the Republican candidate for presidency, John McCain from the two Democrats, Senator Barack Obama and Senator Hillary Clinton. Senator Obama declared that as commander in chief he would certainly withdraw most of the troops in Iraq, leaving only the number required to maintain security for the American embassy and prevent the spreading of terrorism.