"Left unaddressed, the growing instability in Libya could ignite wider instability in the Middle East, with dangerous consequences to the national security interests of the United States," Obama said in a letter to Republican House Speaker John Boehner and Senate President Pro Tempore Daniel Inouye.
The president also answered lawmakers publicly pressing him to detail what US forces aimed to achieve in Libya and openly worrying that "mission creep" could lead to a protracted entanglement with no defined end-point.
Obama said US strikes on Libyan air defenses and military airfields "will be limited in their nature, duration, and scope" and aimed to set the stage for a no-fly zone as called for under a UN Security Council resolution.
"United States forces are conducting a limited and well-defined mission in support of international efforts to protect civilians and prevent a humanitarian disaster," he said.
The US president asserted that "the United States has not deployed ground forces into Libya" but did not repeat past pledges not to deploy them in the future.
And he slammed Gaddafi's "illegitimate use of force," accusing him of killing "substantial numbers of civilians" and "forcing many others to flee" to neighboring countries, thus threatening to destabilize the region.
"Gaddafi's defiance of the Arab League, as well as the broader international community moreover, represents a lawless challenge to the authority of the Security Council and its efforts to preserve stability in the region.
"Gaddafi has forfeited his responsibility to protect his own citizens and created a serious need for immediate humanitarian assistance and protection, with any delay only putting more civilians at risk," said Obama.