If the breaking news late last night that Saudi Arabia had initiated airstrikes against Houthi rebels in Yemen wasn't stunning enough, the announcement that they had also formed a coalition of nine other countries to help restore order there was even more significant.

In a public announcement made just before the first airstrikes were carried out, Saudi Arabia's Ambassador to the United States, Adel Al-Jubeir, declared that the kingdom would do whatever was necessary to restore the Hadi government to power in Yemen.

Officially, the airstrikes were carried out in direct response to a formal request by Abd-Rabbu Mansour Hadi's foreign minister earlier this week for Gulf Cooperation Council (GCC) military assistance in fighting the Houthis.

But for Saudi Arabia, the decision to form a coalition to take military action in Yemen has less to do with supporting the Hadi government than it does with providing diplomatic cover to take on what it considers to be the its biggest external threat - an increasingly aggressive Iran determined to firmly establish its hegemony in the region.

Strategic rivalry

For more than 30 years, the Saudis have been engaged in a strategic rivalry with Iran for power and influence in the Middle East. Built largely upon sectarian and ideological lines, with Saudi Arabia as leader of the Sunni Arab world and Iran as the leader of the Shia crescent, it has been played out in a series of hot and cold proxy wars throughout the region - in Lebanon, Syria, Iraq, Bahrain and now Yemen.

But Iran's extension of that influence into Yemen, vis-a-vis the Houthis, was a line the Saudis could no longer tolerate. For years, claims that Iran was smuggling weapons to the Houthis during their on again-off again conflicts with Yemeni military forces were subject to much debate. But since the Houthis took control of the capital city of Sanaa last fall, Iran's role in their rise has come into much sharper focus.

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Last month, the Houthis signed a civil aviation agreement with Tehran for regularly scheduled flights between the two capitals. That was significant for two reasons. First, it reduced whatever international isolation the Hadi government and CGG countries had hoped to impose on the Houthis.

But more importantly, it strengthened Iran's hand in Yemen, making direct military assistance for the Houthis or the conduct of intelligence operations there that much easier. Case in point: three weeks ago, an Iranian diplomat being held hostage in Yemen for more than 18 months - presumably by al-Qaeda in the Arabian Peninsula (AQAP) - was rescued following an intelligence operation by Tehranwidely reported by Iranian state media.

Yemen and Saudi Arabia share a porous 1,500-kilometre border that can easily be infiltrated by those wishing to do the Kingdom harm, whether they are agents of Iran or terrorists cells. With the Houthis in power, the southern half of that border is as open as they or Iran want it to be.

And just last week, an Iranian ship was reported to have unloaded 180 tons of weapons and military hardware for the Houthis at the port of al-Saleef. Both those incidents, along with untold others are sure to have caught the Saudi's attention. But why does it matter?

Yemen and Saudi Arabia share a porous 1,500-kilometre border that can easily be infiltrated by those wishing to do the Kingdom harm, whether they are agents of Iran or terrorists cells. With the Houthis in power, the southern half of that border is as open as they or Iran want it to be.

Iranian proxy

Consider also, that Iran has significantly strengthened its position in the region over the last several years at Saudi Arabia's expense, particularly in Iraq. For that very reason, Saudi Arabia, which looks at the Houthis as nothing more than an Iranian proxy, has no choice but to draw the line on their southern border.

Which brings us back to the Saudi-led military campaign in Yemen, now known as "Operation Decisive Storm". With its nine coalition partners - Morocco, Egypt, Jordan, Sudan, Pakistan, Qatar, Kuwait, Bahrain, and United Arab Emirates - the Saudis have put together a formidable force, at least on paper.

Some of those countries are reported to have already taken part in airstrikes against the Houthis, while others have deployed combat ships to the region. But, as history has shown time and again, it takes more than air and sea power, no matter how daunting, to rout an army.

If the Houthis are to be defeated, and in turn Iran's influence in Yemen neutralised, ground troops will eventually have to be committed. If that happens, it's important to keep in mind that the Houthis, who have already proven themselves as an able fighting force, will be backed up not only by the Iranians, but also by those military forces that remained loyal to Ali Abdullah Saleh, Yemen's former president who is now firmly in their camp.

And that brings up the very real possibility of not only a costly and lengthy force-on-force engagement, but also a deadly insurgency campaign similar to what has been experienced in Iraq, Syria, Libya and elsewhere. Those are the kinds of challenges that take a heavy toll in both civilian and military casualties, and cause coalitions to rapidly dissolve.

Only hours into the campaign, it's too early to tell what will happen next. But for Saudi Arabia, this will be their first test in leading a coalition to war - and just as importantly, in holding it together. It may also be their last chance to stop Iran on their southern border.

Martin Reardon is a senior vice president with The Soufan Group, a New York-based strategic security and intelligence consultancy, and senior director of Qatar International Academy for Security Studies. He is a 21-year veteran of the FBI, and specialised in counterterrorism operations.

The views expressed in this article are the author's own and do not necessarily reflect Al Jazeera's editorial policy.