"...how is it in the United States' national interest to intervene in Syria's civil war?"
War mongering is a highly profitable enterprise for the companies who produce armaments and supply services in war zones. Raytheon and Halliburton come to mind.
The US is controlled and owned by the El-ite military-industrial-bankster class whose lobbyists, often ex-politicos, have bought off congress. Added to this are the extremely powerful pressure groups such as AIPAC and the Neoconservatives. All of the above cross boundaries and stand together thereby multiplying their sway.
The end result being a totally corrupted nation whose people have been eviscerated of any power to change or influence the direction of their government.
The US Constitution has been smashed in recent times resulting in presidential powers now equaling that of a tin-pot dictator via "executive order", which allows the President to bypass approval from Congress on any matter he so chooses, including going to war.
