This has long been a point of vulnerability for Saudi Arabia:
When Sunni suicide bombers belonging to an Islamic State cell targeted mosques of Saudi Arabia's minority Shi'ite community last month, the government quickly pledged national unity across sectarian lines and offered compensation.
The two blasts killed 25 people, stirring divisions in a kingdom dominated by the ruling Al Saud family and a majority Sunni population that makes up its support base and includes clerics from the strict Wahhabi school who often say Shi'ites are not true Muslims.
Yes, this was ISIL (the Islamic State, as they call themselves now), which is a Sunni Arab terror group. But it is so convenient for Iran that I wouldn't rule out Iran's involvement in clearing a path for the jihadis to strike.
Or even if uninvolved, they simply might exploit this event. If Arab Shias feel under assault in Saudi Arabia, they might turn to Shia Iran for protection.
It would probably help if the Saudis attempted to support dissidents inside Iran to return the favor.
On the bright side, if Iran hopes to scare Saudi Arabia by threatening missile attacks on Saudi oil infrastructure, this will damage that hope:
Saudi Arabia said it shot down a Scud missile fired by Yemen's Shiite rebels and their allies early Saturday at a Saudi city that is home to a large air base, marking a major escalation in the monthslong war.
The Saudis are showing their air force can bomb targets and their Patriot missiles can shoot down ballistic missiles. So a battle to the death of oil exports will be worse for Iran.