When the US, after decades of engagement, leaves the Middle East (if they are foolish enough to do so) they'll be leaving a political power vacuum of Shai-majority citizens for Iran to control by proxy - which doesn't support the US ally of Israel. That, and the Taliban, the Sunni wahabists schools that enabled 9/11, are protected by the Saudis, which the US has been getting cosy with for a while. It could be argued that being friendly with Saudi Arabia by diplomatic means has been more useful in quashing the fundamentalist wahabi jahadist threat, than has been any fighting in either Iraq or Afghanistan.