Bush on Monday delivered the first in a series of planned speeches aimed at convincing Americans he has a strategy for victory, days before the third anniversary of the US-led invasion of Iraq.
Bush spoke amid a wave of sectarian strife in Iraq that has raised fears of civil war, dampening US hopes for a troop drawdown soon.
Bombings in a Baghdad Shia slum killed 52 people on Sunday.
"They're hoping to shake our resolve and make us retreat," Bush told a pro-democracy group in Washington.
"They're not going to succeed."
Bush insisted there had been progress in training Iraqi security forces to take over duties from US troops, but acknowledged there was still plenty of work to be done.
On Monday, at least 14 Iraqis were killed in attacks around the country, including a journalist and a young girl, as police discovered 21 executed bodies.
At least 46 people died and 204
were wounded in Sunday's blasts
An Interior Ministry source said eight corpses were found in Baghdad's Shia slum of Sadr City, the same place a series of bombings killed 46 people and wounded 200 on Sunday.
The bodies of the four had been hung from pylons, residents said. Police said a sign near the bodies, bearing bullet and torture wounds, read: "These are the traitors".
Eleven other bodies, including some blindfolded and with their hands bound, were found elsewhere in the capital and two in the northern city of Mosul, the source said.
While blaming Iraq's sectarian violence on the "enemies of freedom" in the country, Bush on Monday also pointed the finger at Iran, saying some of the homemade bombs that are wreaking havoc in Iraq came from its eastern neighbour.
Locked in a test of wills with Iran over its nuclear ambitions, Bush said during his speech that "some of the most powerful IEDs (improvised explosive devices) we are seeing in Iraq today include components that came from Iran".
Since 2003, more than 2300 US
soldiers have died in Iraq
Remotely controlled explosive devices have taken a heavy toll on US military forces in Iraq and Bush said some of the best minds in America were working on ways to combat them.
There have been more than 2300 US military deaths in Iraq since the invasion in March 2003.
The US now has about 132,000 troops in Iraq.
Quoting John Negroponte, the US national intelligence chief, Bush said Iran had been responsible for at least some of the increasing lethality of attacks in Iraq.
"Coalition forces have seized IEDs and components that were clearly produced in Iran," Bush said.
While defending his Iraq policies and trying to reassure sceptical Americans that he has a coherent plan, Bush gave no sign of when US troops would start coming home.
Bush has consistently refused to set a deadline for a withdrawal. "We will not lose our nerve," he said.