First, Obama intends to persuade Americans that the Syrian regime's use of chemical weapons affects U.S. national security.
Second, he plans to make the case for holding Bashar al-Assad accountable for murdering his own people with chemical weapons in defiance of the longtime international norm.
Third, he'll emphasize that the sort of military action being considered is targeted and limited, with no ground troops and no open-ended commitment. It will be nothing like the wars in Iraq and Afghanistan, Obama will point out.
Fourth, he'll say the threat of U.S. military action has led to negotiations for a possible diplomatic solution that would rid Assad of his chemical weapons and put them under United Nations control. The move, he will argue, brought Russia to the table; it forced Syria to admit it has chemical weapons and signal willingness to sign the treaty against chemical weapons.
Obama will emphasize that the threat of military action remains on the table as the U.S. and other countries pursue a diplomatic solution.