“Any who say that we’ve lost this war, or that we’re losing this war, are wrong. We are not,” Mr Rumsfeld told a hearing of the US Senate Armed Services Committee.
His comments follow stinging rebukes from members of the US Congress, among them members of the governing Republican Party, about the performance of the US military in Iraq.
Earlier this week, top Republican Senator Chuck Hagel slammed the Bush administration’s Iraq policy as “completely disconnected from reality.”
Republican Representative Walter Jones is leading calls for a deadline on when US troops will return home and introduced legislation earlier this month demanding a timetable be set.
But Mr Rumsfeld told the Senate Committee the idea of a timetable would be counterproductive and embolden insurgents.
“It would throw a lifeline to terrorists, who in recent months have suffered significant losses and casualties, been denied havens and suffered weakened popular support.”
Operation Lightening, launched in late May, represents the strongest recent crackdown on insurgent militants.
More than 1,000 suspects were detained.
The US military has just wrapped up a five-day offensive against insurgents in the town of Karabilah, near the Syrian border.
According to officials, 47 militants were killed and one suspect detained.
However, the past two days have seen a deadly string of bombings of Shi’ite targets in the capital killing at least 35 people.
In the latest attacks, four bombs in quick succession were detonated in the densely populated Baghdad district of Karradah.
Two mosques, a police patrol and a male bath complex were hit by the blasts which left 17 people dead and 69 injured, according to an interior ministry official.
Amid the fresh violence, Mr Rumsfeld and leading US military officials have repeated criticisms made by US Secretary of State Condoleezza Rice against Syria.
‘It is a fact that terrorists come across the Syrian border. It is also a fact that Syria is a dictatorship with a very large intelligence community. And one has to presume they know what is going on in their country,” Mr Rumsfeld stated.
“In addition there are Baathists that came from Iraq and have established themselves n Syria, and the Syrians have not apparently taken any action against them as they support the insurgency,” said General John Abizaid, the commander of US forces in the Middle East.