Shifting to foreign policy after two days of hammering the Illinois senator over their differences on health care, Clinton paired two of Obama's campaign statements to support her conclusion.
"He wavers from seeming to believe that mediation and meetings without preconditions can solve some of the world's most intractable problems to advocating rash, unilateral military action without the cooperation of our allies in the most sensitive part of the world," Clinton said in a speech at The George Washington University.
The former first lady has sharply criticized her rival for the Democratic presidential nomination for saying during a televised debate last summer that he would be willing as president to meet with the leaders of Cuba, Iran and other hostile nations without preconditions. He reiterated that willingness last week.
"We simply cannot legitimize rogue regimes or weaken American prestige by impulsively agreeing to presidential talks that have no preconditions," Clinton said. "It may sound good, but it doesn't meet the real world test of foreign policy."
Obama has also said he would be willing to send U.S. troops into Pakistan if there were "actionable intelligence" that the country is harboring terrorists.
Anticipating Clinton's criticism, Obama's foreign policy advisers held a conference call with reporters before she delivered her speech. Top Obama adviser Susan Rice said the New York senator had shown poor judgment on a range of issues, including voting to authorize the invasion of Iraq and supporting legislation declaring the Iranian National Guard as a terrorist organization.
"Those are critical foreign policy judgments. They are judgments that any candidate should be held accountable for. And obviously we look forward to Senator Clinton's explanation of how and why she got those critical judgments wrong," Rice said.
Hoping to slow Obama's surging candidacy before primaries in Texas and Ohio next week, Clinton painted a picture of a dangerous world in need of seasoned and wise U.S. leadership. She portrayed Obama as a national security novice and suggested he would need a "foreign policy instruction manual" to keep the country safe.
Once again, she compared her Democratic rival's foreign experience to that of President Bush upon taking office eight years ago.
Voters have already seen the "tragic result" of electing a commander in chief with little experience in national security and global affairs, she said. "We can't let that happen again. America has already taken that chance one time too many."
On a campaign trip for her through Ohio, her husband, former President Bill Clinton, urged undecideds among a crowd in Portsmouth to vote for her "if you believe that the fact of change is more important than the feeling of change."
Before her speech in Washington, Clinton was introduced by a group of distinguished military veterans who praised her work on international challenges in the Senate Armed Services Committee and as first lady during her husband's White House years.
"She's walked the walk in addition to talking the talk," said Togo West, who served as Army Secretary and Veterans Affairs Secretary during the Clinton administration.
As she has in the past, Clinton vowed as president to begin redeploying U.S. troops from Iraq within 60 days of taking office. She also pledged to address the global AIDS epidemic, work with other nations to reduce global warming, and tackle the challenges associated with the growth of China.