* Obama cites fears that Gaddafi could commit atrocities
* Speech sets stage for another U.S. military involvement
By Steve Holland and Patricia Zengerle
WASHINGTON, March 18 (Reuters) - President Barack Obama laid the rhetorical groundwork on Friday for potential U.S. military involvement in Libya, something he had resisted until agreeing to it this week.
Obama said in a White House speech he was convinced of the need for action in Libya by fears that Libyan leader Muammar Gaddafi would commit atrocities against his own people, kill thousands and destabilize an entire region.
His speech set the stage for U.S. involvement in a third conflict -- Iraq, Afghanistan and now Libya. It marks a fresh military engagement for Obama, who inherited the other two wars from his predecessor, George W. Bush.
Obama had messages for four separate audiences: Gaddafi, European allies, the U.S. military and American citizens.
* To Gaddafi, Obama minced no words with the long-time adversary of the United States: Stop your troops from advancing on the rebel stronghold of Benghazi; pull them back from other contested areas; establish water, electricity and gas supplies to all areas, and allow humanitarian aid to flow.
But he did not repeat past statements that Gaddafi must give up power.
"If Gaddafi does not comply with the (U.N. Security Council) resolution, the international community will impose consequences, and the resolution will be enforced through military action," Obama said.
* To European allies, Obama underscored his determination to work through diplomatic channels as part of an international coalition, a point he has made throughout the Libyan crisis. Obama won election in part in 2008 by denouncing Bush for going ahead with the Iraq war without firm U.N. backing.
Before Thursday's U.N. Security Council resolution, Obama had come under criticism from some conservatives for ceding a global leadership role to Britain and France.
But the United States believes that by taking a lower profile in the diplomacy leading to the U.N. resolution, it allowed Arab states to coalesce around a call for action that provided a impetus to the negotiations. It may have denied Gaddafi the chance to argue that the United States was leading another invasion on Arab soil.
* Obama sought to sketch a limited role for the U.S. military, already stretched thin by the Iraq and Afghanistan conflicts. But he provided few details other than to say no U.S. ground troops would be deployed. He is likely to have more work to do to convince skeptical commanders.
Bruce Riedel, a former CIA Middle East expert who advised the Obama White House on U.S. policy in Afghanistan and Pakistan said: "The administration has not made a compelling case for why we need to go to war for Benghazi so far."
Without giving specifics, Obama said U.S. forces "will provide the unique capabilities that we can bring to bear to stop the violence against civilians, including enabling our European allies and Arab partners to effectively enforce a no-fly zone."
* To skeptical Americans, Obama laid out a rationale for why U.S. interests are at stake and, mindful that the U.S. public is weary of funding wars that seem to have no end, he made clear that "we will not be acting alone."
"Here is why this matters to us. Left unchecked, we have every reason to believe that Gaddafi would commit atrocities against his people. Many thousands could die. A humanitarian crisis would ensue. The entire region could be destabilized, endangering many of our allies and partners," Obama said.
Polls of Americans taken last week showed more than half of respondents opposed U.S. action on Libya, with much smaller numbers supporting it. (Additional reporting by Caren Bohan and Mark Hosenball; editing by Christopher Wilson)
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