How Iran and Saudi Arabia's diplomatic breakthrough could impact the entire Middle East

Last week, Iran extended an official invitation to Saudi Arabia's King Salman to visit Tehran, one of many steps recently undertaken by the two countries as they slowly improve their relations. The two countries have been bitter rivals for decades, but their relationship has begun to thaw since they signed an agreement to resume diplomatic ties in Beijing last month. It was a diplomatic breakthrough in a standoff that has shaped events and attitudes across the Middle East for more than 40 years. 

The two countries completely severed diplomatic ties after Sunni Muslim-ruled Saudi Arabia executed a prominent Shiite Muslim cleric in 2016. Iran's ruling clerics, and most of its population, are Shiite, and the two nations' animosity has exploded into "proxy wars" in several countries across the region, including Yemen, where groups backed by either side have taken a heavy toll in human lives and caused a dire humanitarian crisis.