The Saudis clearly understand this threat. Currently, they are building an extremely fortified 600-mile wall along the Iraq border to protect themselves from an ISIS invasion. A similar wall is being built along the Yemeni border to shield themselves from a possible Houthis Shia invasion from Yemen. It is also the reason that Saudi Arabia has created a coalition of 10 countries to restore Sunni control of Yemen and beat back the Shia Houthis.
At issue, now, is whether or not that same coalition will eventually turn on Iran in an attempt to keep them from ever getting the bomb. In fact, Obama's nuke deal may have just solidified that very possibility. But, even if the Saudis don't lead a war against Iran, there is a very real possibility that the President's deal will create a rush of Sunni countries, lead by Saudi Arabia, to have their own nuclear weapons as a matter of self defense.
Simply, there has never been a worse time for peace in the Middle East because the United States has created a vacuum for ISIS and Iran to walk all over Sunni countries in the region. It is possible that in less than two years, an all out Middle East war could ensue, or as rumored, the Saudis may allow Israel access to their airspace to launch a defensive attack on Iran's nuclear facilities. A fact that previously would have never been possible had Iran not been given a pathway to the bomb under the current nuke deal.
Criticism of Twelver Shia Islam: http://en.wikipedia.org/wiki/Criticism_of_Twelver_Shia_Islam
Iran Now Controls Four Middle Eastern Capitals: https://www.thetrumpet.com/article/124188.8.131.52/united-states/iran-now-controls-four-middle-eastern-capitals
Saudi Arabia Builds Iraq Border Wall To Protect Against ISIS: http://www.npr.org/2015/01/20/378660034/saudi-arabia-builds-iraq-border-wall-to-protect-against-isis
Saudi Arabia builds giant Yemen border fence - BBC News: http://www.bbc.com/news/world-middle-east-22086231
Saudi Arabia to allow Israel use of its airspace to strike Iran: http://rt.com/news/235923-saudis-airspace-israel-iran/
Sudanese army says ground troops to join Saudi-led coalition in Yemen: http://www.sudantribune.com/spip.php?article54415
Turkey-Pakistan reaffirm support for Saudi Arabia: http://www.worldbulletin.net/turkey/157367/turkeys-domestic-security-law-enters-into-force