What countries have gotten involved in the Sunnis vs. Shiites conflict?

Answer 1

It is incorrect to say that there is an international Sunni/Shiite conflict. There is no general conflict between all or almost all Sunnis and Shiites in the way that there were conflicts in Europe between almost all Catholics and Protestants in the 16th and 17th century (Thirty Years War et al.). While there are certainly conflicts between Sunnis and Shiites, it is always in a localized or national context, not an international one. In much of the Muslim world, "Sunni" and "Shiite" are viewed as more than just religious labels, but have a "quasi-ethnic" component. It has the power to determine social groups and political organizations, prompting conflict on those grounds.

However, typically, the various conflicts across the Middle East where Sunnis and Shiites fight one another (such as the Iran-Iraq War, the Lebanese Civil War, the Iraqi Insurgency, the Bahraini Uprising, and the Syrian Civil War) other large Islamic States and Western States enter the fray. Saudi Arabia, several Gulf States, and Jordan usually support Sunni militants. Iran and Syria tend to back Shiite militants. The Western Powers, especially the United States, United Kingdom, France, and Germany tend to support whichever side of the conflict purports to agree more with their idelogies of how the region should be governed or to avenge political grievances with an Islamic government with whose policies it disagrees.