For decades, any list of global geopolitical risks will have had "attack on Saudi oil facilities" near the top. Now it has happened.

The good news is that the world is less vulnerable to an oil price shock than it was in the 1970s, when the Opec oil embargo created turmoil in the global economy. It is also true that all of the major powers involved - Saudi Arabia, Iran and the US - have strong incentives to avoid an all-out conflict.