In her first campaign swing through this early nominating state, the New York senator told party activists that Democrats in 2008 will face "someone on the other side who will be very tough and strong, even bellicose perhaps."
That likely was a reference to Arizona Sen. John McCain, who has taken a hard line in supporting more U.S. troops to Iraq, as President Bush has announced.
The former first lady also said has learned the lessons of the last two presidential campaigns, both lost by Democrats who responded slowly to criticism.
"When you are attacked, you have to deck your opponent," Clinton said. "I have been through the political wars longer than some of you have been alive. We've got to be prepared to hold our ground and fight back."
Clinton, who announced her candidacy last weekend, said Democrats cannot concede the security issue.
"We have to nominate someone who can have the trust and confidence of the American people to make the tough decisions as commander in chief," the former first lady said. "That is the threshold issue."
Her initial foray in Iowa was far different from the traditional caucus campaigning, with a few people in a living room. More than 1,500 people jammed a high school gymnasium for a town hall-style meeting. Some 150 reporters and photographers chronicled the event.
Clinton knows she could be counted out if she doesn't win in Iowa, reports CBS News correspondent Cynthia Bowers. Candidates who stumble in either Iowa or New Hampshire find it hard to regain the momentum they need to win their party's nomination.
"If somebody's able to become the clear winner in Iowa and New Hampshire, frankly, no matter when those states' primaries are, no matter how big they are, I don't think they'll be able to buck the trend that is set in Des Moines and in Manchester," Peter Greenberger, a Democratic political consultant, told CBS.
Earlier, Clinton met with state Democrats at the party's headquarters.
Attention focused on Iraq and her vote to authorize the use of force ahead of the U.S.-led invasion in March 2003. Presidential rivals such as former North Carolina Sen. John Edwards now say the vote in support was a mistake.
"There are no do-overs in life," Clinton said. She says Congress received bad information going into the vote and that she would have voted differently given what she knows now.
"As a senator from New York, I lived through 9/11 and I am still dealing with the aftereffects," Clinton said. "I may have a slightly different take on this from some of the other people who will be coming through here."
Clinton said her view was that the nation was engaged in a deadly fight against terrorism, a battle that she contends President Bush has botched.
"I do think we are engaged in a war against heartless, ruthless enemies," she said. "If they could come after us again tomorrow they would do so."
Clinton has urged a cap to the number of U.S. troops in Iraq, but has refused to go along with suggestions that Congress use its power of the purse to bring the war to a halt.
"This will be a problem that will be left to the next president," the senator said. "We've got to figure out now, given where we are, how we go forward."
Seeking a "phased redeployment" of troops from Iraq, she said, "We've got to bring the Iraq war to the right end." The Democratic-controlled Congress, she said, must start to "build the political will" to stop the president.
Clinton joked about the emotions she stirs in both those who like her and those who do not. "I know what I'm getting into. I do inspire strong feelings," she said.
She later planned to visit eastern Iowa for house parties in Cedar Rapids and Davenport.