Shia Cleric Nimr al-Nimr, the prominent religious leader of a Shia minority in Saudi Arabia, was executed by Riyadh. This event caused a great protest of Iranians that attacked the Saudi embassy in Tehran and the consulate in Mashhad and also a more global emotion in the world.
In fact Shite Iran and Sunni Saudi Arabia have been rivals in the region for a long time. So, this execution has started to destabilize the relations between Iran and Saudi Arabia and at the same time, it has reaffirmed the importance of the Iranian nuclear deal.
Saudi Arabia used the unjustifiable arson of the Saudi embassy in Tehran as a pretext to cut the diplomatic ties with Iran. In this situation,US has expressed its concern and Saudi Arabia is trying to involve Washington to balance the relations between Iran and Saudi Arabia.