US Defence Secretary Donald Rumsfeld has paid a surprise, morale-boosting seasonal visit to US troops in Iraq.
Rumsfeld is keen to express his concern for US troops in Iraq
Despite admitting "setbacks", he told soldiers they should not doubt their ability to win the war.
The first three stops of Mr Rumsfeld's visit took him to the frontline in the struggle - Mosul, Tikrit and Falluja, once Saddam Hussein's top strongholds.
Recent clashes in Falluja have proved the bloodiest since the invasion, while an attack in Mosul this week killed 22.
The visit comes amid mounting criticism of the defence secretary, after his recent admission that he used a machine to sign condolence letters to the families of soldiers killed in Iraq.
But Mr Rumsfeld insisted the trip had been planned for a while, and was not in direct response to Tuesday's blast in a crowded dining hall at the US base.
Thirteen US troops, five US civilian contractors, and four Iraqis died in the attack, the worst so far against Americans since the removal of Saddam Hussein.
Another 70 Americans were wounded. Most of them have now been airlifted to US military hospitals in Germany.
In other developments:
- One person is killed and at least 19 hurt after a tanker explodes in an upmarket Baghdad neighbourhood, near several foreign embassies
- Gunmen reportedly blow up the mayor's office and a police station in the city of Ramadi, but there is no word of casualties
- US Brigadier General Richard Formica is appointed to investigate how a suicide bomber penetrated the Mosul military base
- Iraqi government officials urge residents of Falluja to return to the devastated city, which was recently retaken from insurgents
- Insurgents attack a police station and official buildings in the Baquba area, north of Baghdad
In Mosul, the first leg of his one-day visit, Mr Rumsfeld acknowledged the situation looked "bleak" for US troops in Iraq.
But he added they would ultimately win in what he described as something "truly historic".
RUMSFELD'S ROUGH MONTH
17 Dec: Admits using a machine to sign Iraq condolence letters
Mid-Dec: Criticised by top Republicans Trent Lott and John McCain
8 Dec: Grilled by US troops over Iraq equipment
Addressing a gathering of marines who took part in last month's major offensive against insurgents in Falluja, Mr Rumsfeld later said: "All along the way, it's bumpy and it's tough... It is not a smooth, easy path to success - there are setbacks."
"But you will look back when you are about my age, and you will be proud," he added.
"When it looks bleak, when one worries about how it's going to come out, when one reads and hears the naysayers and doubters who say it can't be done and that we are in a quagmire here now.
"The fact is there have always been people throughout every conflict of the history of the world who said it couldn't be done.
"And people will be able to look back and know that they've been involved, each of you will be able to look back and know that you've been involved in something truly historic."
Mr Rumsfeld also visited the base's hospital, where he was photographed with injured troops and handed out bronze Pentagon coins.
During his final stop in Baghdad, he met the top commander in Iraq, General George Casey, as well as interim Iraqi President Ghazi Yawar.
Speaking about the attack on the dining hall tent in Mosul, Gen Richard Myers, chairman of the US Joint Chiefs of Staff, said "an improvised explosive device worn by an attacker" was the most likely cause of the explosion.
The US military says the bomber was wearing an Iraqi military uniform.
Mr Rumsfeld has faced questions about how security at the Mosul base could have been breached and troops left vulnerable.
The defence secretary said on Wednesday he was truly saddened by claims that he was failing in his duty to protect the troops.
"I hope and pray that every family member of those who have died so bravely knows how deeply I feel their loss," he said.
Senior Republicans have been openly questioning President George W Bush's decision to keep Mr Rumsfeld at the Pentagon for his second term.
On 8 December, the defence secretary received a public grilling from US soldiers in Kuwait, who alleged that scrap metal was being used to armour vehicles employed in Iraq.
Mr Rumsfeld has made a number of such trips since the US-led invasion of Iraq, but the BBC's Michael Buchanan in Washington says this is arguably the most important one.