When Ronald Reagan, the last American president to bomb Libya, launched military operations over the skies of Tripoli, he addressed the nation from the Oval Office. When George H.W. Bush launched military operations in Panama, the Persian Gulf and Somalia, he addressed the nation. So did Bill Clinton when he launched military strikes in Iraq, Afghanistan, Sudan and Yugoslavia. So did George W. Bush when he launched military operations in Afghanistan and Iraq.

Now here we are, after 10 days of combat in Libya, and only Monday night will Barack Obama finally deign to speak directly to the American people about the military action he has ordered. Obama has spent more time and effort winning the support of the United Nations and the Arab League than he has rallying the American people or their representatives in Congress. Result? A Gallup poll last week found that just 47 percent of Americans approve of Obama’s intervention in Libya — the lowest level of support of a U.S. military campaign in at least 30 years. Little wonder. The president has not explained the reasons for the military action he ordered, the objectives of the mission and the stakes involved, or given Americans a reason to support him. Let’s hope Monday night he does.

This problem is not just limited to the military mission in Libya. The Post recently reported that support for the military mission in Afghanistan is collapsing, with nearly two-thirds of Americans saying that the war in Afghanistan is not worth fighting — the highest proportion yet opposed to the conflict. When George Bush left office, a solid majority of Americans said the war in Afghanistan was worth it. That support has dropped by 21 points on Obama’s watch.

What happened? Obama has failed to lead. When is the last time you heard Obama deliver a major speech making the case for his policies in Afghanistan? He did so in December 2009, announcing the surge of forces, and the public rallied to his cause, with support rising back above 50 percent for the first time in six months. Then Obama went radio-silent again — and support for the mission plummeted to an all-time low of 34 percent.

Why won’t Obama speak out for his own policies? For one thing, the military actions he has ordered are deeply unpopular with his liberal base (only 19 percent of Democrats polled say the war in Afghanistan is worth fighting). But if the president cared, he would take the risk of alienating key constituencies he needs for reelection to defend his decisions. That is what he did in the health-care debate, barnstorming the nation to persuade Americans to support his unpopular bill. When he was asked why he risked alienating independent voters, Obama famously replied, “I’d rather be a really good one-term president than a mediocre two-term president.” Why doesn’t this same principle apply to national security?

On Libya, Obama has not only failed to rally the American people, he has also failed to rally the world. In his radio address last weekend, Obama patted himself on the back for letting others take the lead, declaring, “This is how the international community should work: more nations, not just the United States, bearing the responsibility and the cost of upholding peace and security.” More nations? According to Foreign Policy magazine, so far the international coalition confronting Moammar Gaddafi is smallest of any major multilateral military operation since the end of the Cold War — with only 15 countries, including the United States, participating (compared to 32 countries in the Gulf War; 34 in Bosnia; 19 in Kosovo; 48 in Afghanistan; and 40 in Iraq). The world won’t follow if the American president does not lead.

The vacuum Obama has left on the international stage is being filled by French President Nicolas Sarkozy, who has seized the initiative on Libya, recognizing the opposition, organizing the first international conference to coordinate the world’s response, and leading the effort the secure a U.N. Security Council resolution. When Obama declared Gaddafi must go but did nothing about it, French Foreign Minister Alain Juppe scored him in his blog, declaring: “It is not enough to proclaim, as did almost all the major democracies, that ‘Gaddafi must go.’ We must give ourselves the means to effectively assist those who took up arms against his dictatorship.” The initiative to do so came from the Elysee Palace, not the White House.

Obama may be the first president and commander in chief in modern times to voluntarily cede another title comes with his job: leader of the free world. Not only has he ceded this title, he has ceded it to the French. Incroyable.

Marc A. Thiessen, a visiting fellow with the American Enterprise Institute, is the author of the book “Courting Disaster” and writes a weekly column for The Post.