Iranian and Saudi proxies are already at war across the Middle East. Syrian President Bashar al-Assad, a close Iranian ally, has received money and weapons from Tehran, which has also dispatched intelligence operatives and combat-hardened fighters from Lebanon’s Hezbollah militia to Assad’s aid. Meanwhile, Riyadh has funneled weapons, money and supplies to the loose-knit alliance of rebels fighting to unseat him. The outside support keeps the country’s brutal civil war chugging along, with hundreds of thousands dead, millions displaced, ancient cities such as Aleppo largely obliterated and neither side closer to a final victory.

In Lebanon, Iran has built Hezbollah into the strongest paramilitary force in the region, one capable of repeatedly beating back the mighty Israeli army. Hezbollah is also the most powerful political party in Lebanon, where it holds veto power over the country’s fragile, pro-Western, Sunni-led central government. Riyadh, hoping to prop up Lebanon’s official military as a bulwark to Iran, announced last year that it was giving that force an eye-popping $3 billion grant, nearly double its annual $1.7 billion budget. Beirut’s troops aren’t nearly as well-equipped or well-trained as Hezbollah’s, but the Saudi money will help narrow that gap. Iran responded with an aid offer of its own, pledging to give the Lebanese military — and, significantly, Hezbollah — ammunition, mortars, anti-tank missiles and other light weaponry.
Elsewhere, Bahrain’s government believes that Iran is fomenting unrest among the country’s Shiite majority in an attempt to force regime change in the oil-rich Sunni monarchy. Saudi Arabia deployed troops there in 2011 to help quell a Shiite uprising.