Is the United States a Sunni or Shiite Nation?
The United States does not have an official religion despite the desire of the religious right to make it so. Arguably the U.S. was born out of Judea-Christian principals but according to the Constitution all citizens have the Freedom of Religion and more importantly Freedom from Religion.
Which is why a question of what Muslim ideology the U.S. follows is facetious at best yet the U.S. find itself somewhat playing both sides of a holy war that has been raging for close to two millenniums.
While the U.S. sides with the Sauds who are Sunnis, that is the believe that the ruler of all Islam should be a Caliph or king that is chosen by the Ummah or community. It is the Wahabi sect of Sunni Sauds that spawned Osama Bin Laden and Al Qaeda not to mention the Islamic State ISIS which too is a vicious Sunni force.
To contrast the Sunnis is the Iranians and the Assad regime of Syria who are of the Shiite sect, those who believe Islam should be ruled by the descendants of Ali, the son-in-law of Muhammad.
So while the Sunni Islamic State run amok , reeking havoc throughout Syria and Iraq, Shiite forces armed and financed by Iran and Hezbollah push-back against ISIS perhaps with more success than the U.S.
This puts the U.S. an its Western allies in a bit of a diplomatic pickle for as much as they wish to destroy ISIS the monster they created they also wish to topple Assad from power and crumble any chance of a nuclear developing Iran. In this case “the enemy of my enemy is still my enemy” to the United States and its convoluted messy politics.