NATO has escalated its bombing campaign around the Libyan capital of Tripoli in an effort to bring the conflict to a close and has destroyed one of Muammar al- Qaddafi's favored retreats . A senior NATO military official told CNN on June 9 that Qaddafi was a legitimate target of the bombing campaign, but declined to comment on whether he was being deliberately targeted. Is it legal to deliberately try to kill the leader of a sovereign state? The law is vague. The long- standing reluctance of militaries to engage in the targeted killing of heads of state is based more on custom than codified regulation. ( It's not really in the interest of presidents and prime ministers for that sort of thing to become common practice.) The closest thing in international law to a ban on assassination is the 1907 Hague Convention on the laws of war, which prohibits signatories from attempting "To kill or wound treacherously individuals belonging to the hostile nation or army." Treachery is a tough thing to prove in court and in any case, might not apply to this situation: Qaddafi has been given ample warning and a clear message that NATO and the United Nations want him out. Moreover, Security Council Resolution 1973 authorizing the Libya intervention allows U.N. member states to "take all necessary measures ... to protect civilians and civilian populated areas under threat of attack in the Libyan Arab Jamahiriya." While coalition forces haven't yet declared Qaddafi an official target as part of these measures, they've also made it clear that his personal safety is not a consideration. As British Defense Minister Liam Fox put it, "There's a difference between someone being a legitimate target and whether you would go ahead with targeting." The official who spoke with CNN described Qaddafi as being part of the "command and control" structure of the Libyan military, meaning that taking him out would fall under the mandate of protecting civilians. What about U.S. law? An executive order signed by President Ronald Reagan in 1981 - - updating an earlier order by President Gerald Ford -- states that "No person employed by or acting on behalf of the United States Government shall engage in, or conspire to engage in, assassination." (The order followed embarrassing revelations of CIA plots to kill foreign leaders such as Congolese President Patrice Lumumba and Cuban leader Fidel Castro.) But the order doesn't define "assassination" and, in truth, hasn't had much effect on U. S. policy. Since 9 /11 , the United States has repeatedly targeted senior al Qaeda leaders for assassination, culminating in May's raid in Abbottabad, Pakistan. Even before the war on terror began, the targeting of foreign leaders was hardly unheard of. Reagan himself targeted one of Qaddafi's compounds during airstrikes on Libya in 1986 , killing the Libyan leader's adopted daughter. U.S. airstrikes targeted Saddam Hussein's compounds during the early days of the Iraq war. The deliberately vague "command and control" language is also not new. In 1999 , following an airstrike on Serbian President Slobodan Milosevic's Belgrade residence, Eric Holder then U.S. deputy attorney general, argued that it did not violate the rule against assassination: "Bombs are dropped on command and control facilities," he said in a news conference quoted by Canada's Globe and Mail . "There has not been any attempt on the part of the United States to target any particular individual." So while the killing of foreign leaders is generally frowned upon and rarely admitted to, Qaddafi probably shouldn't be counting on the law to protect him.