Just war theory provides the moral principles for the declaration and waging of war. Just cause needs to be present, which means averaging a wrong, restoring what was unjustly seized. In the case of Syria, the United States has not been attacked, nor it's national interests at risk.
A country does not have the right to interfere at will in the internal matters of another state, especially to remove a legitimate leader of a government. The lawful authority must declare war and must be acting on behalf of the people. He right intention needs to be there. Some countries want to destroy Syria.