"I can't tell you how strong the enemy will be.I can't predict exactly how things will go," he told members of the Senate Foreign Relations Committee.
"The course of war is simply not something one can determine," Wolfowitz said.
He said US officials remain committed to the goal of helping Iraqi forces assume to role of maintaing security in the country.
"Our goal is to put responsibility in their hands as quickly as we can and not too rapidly, to create problems," Wolfowitz said.
"We can say, I think, with reasonable confidence that we have a plan to train and equip and organize very substantial Iraqi security forces by the end of this year."
But it is unlikely they will be ready to take on Iraq's security challenges alone, he said.
"How much they will still need help from the United States, I can't predict. We want it to be as little as possible," said Wolfowitz.
"We're in this to win, as I think you agree, and winning means having the Iraqis take as much responsibility as they possibly can, but also not putting them so far out in front that they fail."