An Iowa State University professor says he thinks the United States’ objectives as the leader of NATO attacks on Yugoslavia are clear. PoliticalScience professor Stephen Schmidt says by leading the way in hitting Serbian forces threatening Kosovo the U-S maintains its world credibility by following through on its threats of action. He says if the U-S is going to be a superpower it has to act like one. Schmidt says these attacks are part of the overall strategy of picking where and when the U-S will take action on the world stage. Defense Department officials say U-S warships and B-52 bombers have fired cruise missiles against Serb air defense targets in Yugoslavia. Targets included missile batteries and other sites in and outside of Kosovo.