MARSHALLTOWN, Iowa–Calling it “Hillary’s war,” Sen. Rand Paul (R., Ky.) told voters in this politically potent state that the U.S. intervention in Libya has been an “utter disaster” that empowered radical Islamist groups. Mr. Paul, on a swing through the state that holds the first nominating contest of the presidential election, argued that U.S. foreign policy in Libya, Syria and elsewhere had helped create threats such as Islamic State, among other groups. Mr. Paul said Hillary Clinton was to blame for what he described as foreign-policy failures, previewing lines of attack for a potential general election match-up. Mrs. Clinton served as secretary of state under President Barack Obama until 2013 and was a proponent of interventions during popular uprisings against the ruling regimes in Libya and Syria. “Hillary’s war in Libya has been an utter disaster,” Mr. Paul said Saturday, speaking to a small group of voters and activists. “There are now jihadists roaming all across Libya. It’s a jihadist wonderland.” Under Mr. Obama, the U.S. government was part of an international coalition to oust Libyan leader Moammar Gadhafi from power in 2011. In Syria, the U.S. has expressed support for the ouster of Bashar al-Assad and has provided both humanitarian and lethal aid to some Syrian opposition groups. In the aftermath of Mr. Gadhafi’s ouster, Islamist groups operating in Libya staged an attack in 2012 on the United States consulate in Benghazi, killing four Americans, including U.S. ambassador Christopher Stevens. Mr. Paul said that Mrs. Clinton had failed to provide adequate security for the U.S. mission to that country which, he said, “should forever preclude her from being considered for the presidency.” A request to Mrs. Clinton for comment on Mr. Paul’s remarks wasn’t immediately answered. The administration has reiterated its support for the 2011 intervention in Libya, congratulating the country for holding elections last year. “The United States was proud to support the Libyan people in the darkest days of their revolution and through their efforts to end the Gadhafi regime,” Mr. Obama said in a 2014 statement. Mr. Paul called the former secretary of state the “biggest cheerleader” for intervention in Syria and Libya and said that those policies had empowered radical Islamic groups in both countries. “Gadhafi was a secular dictator,” the senator said. “Not the kind of guy that we want to have representing us in country, but he was secular. He didn’t like radical Islam, and he kept them down because they were a threat to him. What happened when we toppled the secular dictator? Chaos. More radical Islam.” In Syria, Mr. Paul said that Islamic State — a militant group operating in Syria and Iraq that is also known as ISIS — was essentially created by the U.S. aid program under the Obama administration. “I think we have to do something about ISIS, he said. “But, you know why we’re doing something and why we have to be there again? Because of a failed foreign policy that got us involved in a Syrian Civil War . . . . By supporting the Islamic rebels, ISIS grew stronger and stronger and stronger. And now we have to go back.” Mr. Paul, a libertarian-leaning senator, has broken sharply with the more hawkish wing of his party, prompting a major foreign policy debate within Republican circles about American military power and the role of the United States in the world.