From Wikipedia: 'Following this period, Sunni-Shia strife has seen a major uptick, particularly in Iraq, Afghanistan and Pakistan. Many explain the bloodshed as the work of conspiracies by outsiders - "the forces of hegemony and Zionism which aim to weaken [Islam]" (Akbar Hashemi Rafsanjani and Yusuf al-Qaradawi), unspecified "enemies" (Iran president Mahmoud Ahmadinejad), or American neo-liberals who wish to provoke "a debilitating Islamic civil war." (Dilip Hiro).
Others lay the blame for the strife at a very different source, the unintended effects of the Islamic revival. According to scholar Vali Nasr, as the Muslim world was decolonialised and Arab nationalism lost its appeal, fundamentalism blossomed and reasserted the differences and conflicts between the two movements, particularly in the strict teachings of Sunni scholar Ibn Taymiyyah. The Iranian Islamic revolution, changed the Shia-Sunni power equation in Muslim countries "from Lebanon to India" arousing the traditionally subservient Shia to the alarm of traditionally dominant and very non-revolutionary Sunni. "Where Iranian revolutionaries saw Islamic revolutionary stirrings, Sunnis saw mostly Shia mischief and a threat to Sunni predominance."
Although the Iranian revolution's leader, Ayatollah Khomeini, was very much in favor of Shia-Sunni unity, he also challenged Saudi Arabia, in his view an "unpopular and corrupt dictatorship" and an "American lackey" ripe for revolution. In part because Saudi Arabia was the world's major international funder of Islamic schools, scholarships, fellowships, etc., this angered not only Saudi Arabia but its many fundamentalist allies and benefactors throughout the Sunni world.
Olivier Roy, research director at the French National Centre for Scientific Research, sees the "Shia awakening and its instrumentalisation by Iran" as leading to a "very violent Sunni reaction", starting first in Pakistan before spreading to "the rest of the Muslim world, without necessarily being as violent." According to Roy, "two events created a sea change in the balance of power between Shia and Sunnis: the Islamic revolution in Iran and the American military intervention in Iraq" in 2003. "Today, Azerbaijan is probably the only country where there are still mixed mosques and Shia and Sunnis pray together." '
It's incredibly sad to think how much Iraq changed after 1980.