Mr Rumsfeld paid US troops in Iraq a surprise Christmas Eve visit after weeks of controversy over his handling of the war and perceived lack of sensitivity to US forces and their families.
He said the number of US troops deployed in Iraq has been increased to 150,000 ahead of next month's scheduled national elections.
"I don't want to in any way paint a picture that it's pretty because it isn't. This is a tough country," he told troops gathered at their base near the Baghdad airport during the last stop of his trip, summing up the message he delivered to battle-tested US soldiers throughout the day.
Mr Rumsfeld 's helicopter tour took him to some of Iraq's most volatile cities -- Mosul, scene of this week's suicide attack on a US military mess hall; Tikrit, ousted Iraqi President Saddam Hussein's hometown; and Falluja, where US troops battled Iraqi insurgents last month.
In Mosul, Mr Rumsfeld visited staff and patients at the 67th Combat Surgical Hospital, which dealt with the casualties from the attack on Tuesday that killed 18 Americans, the deadliest such attack since the war in Iraq began in March 2003.
Drawing comparisons with Afghanistan immediately after the fall of the Taliban he said he believed Iraq also had potential to make progress.
"When it looks bleak, when one worries 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're in a quagmire here, the fact is there have always been people throughout every conflict in the history of the world who said it couldn't be done," he told the troops.
"People will be able to look back and know that they've been involved ... in something truly historic, something truly important."
Rumsfeld said he had no doubt victory was achievable.
He later flew south to Tikrit, where he was received with applause at one of Saddam's former palaces, before moving on to Falluja where US forces engaged entrenched Iraqi insurgents in the largest military offensive since the end of the war.
"This is a tough situation here in Iraq. It's dangerous, people are being wounded, people are being killed ... The great sweep of human history is for freedom and you are on the right side of that," he told a town hall-style meeting of Marines.
Mr Rumsfeld was greeted respectfully and enthusiastically during his stops but privately several servicemen expressed frustration at the length of their deployment, especially at Christmas.
The trip followed mounting criticism in Washington over Mr Rumsfeld 's stewardship of the US-led war.
He faced calls for his resignation this month after he appeared to dismiss with an offhand remark concerns from Iraq-bound soldiers in Kuwait who told him they had to scavenge for scrap metal to protect their vehicles.