A USA attack on the Syrian regime is not justified. The security of the USA is not threatened by the Syrian regime. However, the USA should for humanitarian reasons, provide maximum support for Syrian refugee populations in Turkey, Jordan , Iraq and Lebanon. Further it is in the political and strategic interest of the USA that Syrian rebel factions that are friendly to the USA, or antagonistic to Iran, win the civil war. Therefore the USA should provide advanced military weapons support (including necessary training) to any such rebel factions it can contact. The USA should try to help any Syrian rebels win the civil war who could counteract Iranian power in the region.
Iran is a true enemy of the USA and any means of reducing Iranian strength is a worthy strategic goal for the USA to pursue.