BREMMER: This is the worst-ever tension between Iran and Saudi Arabia

YemenREUTERS/Nabeel QuaitiA Southern People’s Resistance militant loyal to Yemen’s Saudi-backed President Abd-Rabbu Mansour Hadi mans an anti-aircraft machine gun the militia seized from the army in al-Habilin, in Yemen’s southern province of Lahej March 22, 2015.

Saudi Arabia is bombing Iran-backed rebels in Yemen, and the regional sectarian proxy war is at a fever pitch.

“The Saudis were caught off guard by how quick and aggressive the Houthi offensive was and felt they needed a sharp and immediate response,” geopolitical expert Ian Bremmer, president of Eurasia group, told Business Insider over email. “They didn’t want Iran to think they needed Egypt or anyone else to come rescue the Kingdom. This is the worst tension we’ve seen between Iran and Saudi Arabia, period.”

The Saudi offensive, named “Decisive storm,” is backed by nine countries — including Egypt and the US — and could escalate quickly as Egyptian officials are saying a ground assault is planned.

Elsewhere in the region, the US has reportedly taken the lead from Shia Iran in the Iraqi battle to drive ISIS militants (many of them Sunni extremists) from Saddam Hussein’s hometown of Tikrit.

The US officially says it is backing most Sunni rebels fighting Bashar al-Assad in Syria, but White House policy is actually aligned with the Iran-backed regime as both consider ISIS the largest threat. Saudi Arabia has actively backed Syrian rebel groups over the course of the war, which is now in its fifth year.

Bremmer noted that turmoil in Yemen does not significantly change US calculations elsewhere in the Middle East.

“Yemen conflict doesn’t affect Iraq situation,” Bremmer said. “The US has been leading the anti-ISIS charge and that won’t change. And on Iran negotiations Saudis remain among the strongest opposing the deal but right now a framework agreement looks very likely — I think that will happen.”

In any case, the stakes have rarely been higher regarding relationships between the US and its allies in the region — most of which opposed to Iran’s increasing assertiveness.

Kerry saudiREUTERS/Evan Vucci/PoolU.S. Secretary of State John Kerry (L) attends a news conference with Saudi Arabia’s Foreign Minister Saud bin Faisal bin Abdulaziz Al Saud in Riyadh March 5, 2015. Kerry met Gulf Arab foreign ministers in Riyadh on Thursday to brief them on progress in the nuclear talks with Iran and offer reassurance that any deal would not damage their interests.

NOW WATCH: This 26-year-old from Baltimore took a 35,000-mile road trip and ended up fighting in the Libyan revolution

Business Insider Emails & Alerts

Site highlights each day to your inbox.

Follow Business Insider Australia on Facebook, Twitter, LinkedIn, and Instagram.