Continued Saudi air attacks risk inciting additional unrest in Yemen. The collateral damage and civilian casualties that airstrikes almost inevitably cause will galvanize opinion against Riyadh. In retaliation, the al-Houthis could carry out cross-border attacks or even use long-range Scud ballistic missiles seized from the Yemeni armed forces.
The Saudis, however, are unlikely to be easily deterred and are not without options. Riyadh has the advantage of ample fiscal reserves that will sustain the campaign against the al-Houthis. The Saudis can use these funds to improve Hadi and his supporters' positions in Yemen and to buy influence with tribal factions throughout the country, a tool Saudi Arabia has used in the past. In particular, funding the Sunni tribes in key energy-rich areas of Marib province will help them better repel the al-Houthis. Riyadh's wealth also gives it leverage over the al-Houthi militant factions, which will not be able to run the country without the substantial aid that comes from Saudi Arabia. However, the al-Houthis could secure other financial backers, a prospect that concerns the Saudis.
In addition to money, the Saudi air force is a powerful weapon in its own right. From the advent of the first Gulf War and increasingly since the drawdown from the second, Riyadh has invested hundreds of billions of dollars in its military, and the Saudis have consistently purchased some of the latest equipment, including advanced missiles and guided munitions for their aircraft. While the Saudi air force still faces significant limitations — namely its dependence on foreign contractors for maintenance, the varied and uneven quality of its pilots and its insufficient intelligence, surveillance and reconnaissance capabilities — it is still more than capable of securing the skies over Yemen and projecting force over long distances at tolerable risk.
In addition to the air campaign, the Saudi-led multinational coalition initiated a naval operation focused on blockading Yemen's ports and securing the Bab al-Mandeb Strait. Egypt and Pakistan allegedly sent vessels to assist Saudi naval elements in the area. Blockading Yemen from the sea while Saudi Arabia imposes a no-fly zone will allow the coalition partners to isolate Saleh and the al-Houthis from external material support. Yet the air campaign and the naval blockade will not be enough. Current military operations will severely constrain the al-Houthis, but if Saudi Arabia seeks decisive victory, it will need to deploy ground forces.
While Saudi, Egyptian and other coalition officials have alluded to a potential ground intervention, it is important to consider the different forms it could take. The coalition could deploy small teams of special operations forces to work alongside indigenous tribal factions or Hadi's forces. Or, if fully committed, there could be an all-out conventional operation with heavy armor that requires significant logistics. While Saudi Arabia would like to maintain the smallest possible footprint in Yemen that enables it to still achieve its objectives against the al-Houthis, Hadi's forces appear severely distressed, and a large ground incursion may be necessary if certain conditions are met.
The advantages of going into Yemen with a large ground force are obvious. Doing so would maximize firepower while enabling the coalition to actually seize, hold and transfer occupied ground to its preferred proxies. At the same time, however, a large ground invasion carries its own risks and complications. For one thing, casualties are virtually assured in such a campaign, and potentially high numbers of them. The coalition members would be the invading force and, as the Egyptians found out during the 1960s, they would be fighting in harsh and broken geography against an ideologically motivated enemy that is intimately familiar with the terrain. Making any invasion painful and costly is the al-Houthis' greatest advantage.
Furthermore, once it penetrated Yemen's border, the Saudi-led coalition would encounter more problems. It does not have much training or experience in counterinsurgency warfare, and it risks turning the local population against it. The Egyptians have struggled with counterinsurgency in the Sinai Peninsula, and there is no reason they would perform any better in Yemen. Moreover, such an operation would invite a heavy financial cost and could drag on for years, especially if the al-Houthis abandoned conventional fighting tactics and resorted to a purely guerrilla fighting style that maximizes the benefits of knowing the difficult terrain. Ground forces deployed in Yemen would also be vulnerable to attacks by tribal militias that resent a foreign power, as well as attacks by al Qaeda in the Arabian Peninsula and Islamic State franchises. In the end, Saudi Arabia will make its decision based on the calculation of those constraints and the feasibility of actually achieving its objectives.
The Saudis and their allies displayed solid operational security leading up to the air campaign against the al-Houthis, taking their targets — and the world — largely by surprise. Deploying ground forces will not be as easy to hide, however. For instance, concealing a large body of troops from Egypt or Pakistan moving closer to the combat theater would be difficult, even if naval transports are used. The coalition would also likely bombard enemy ground forces before invading, another obvious indicator of an impending offensive. The Saudis could choose the least visible option by launching an independent ground campaign from the north, but because it is the most obvious route across heavily mountainous terrain that is controlled by the al-Houthis, such a course of action would be the most difficult and the easiest to defend against.
But the Saudis may be able to disrupt their enemies without actually launching a ground incursion. Just the threat of an attack could distract or fix al-Houthi forces that have been driving south to fight the government troops. Indeed, since the Saudi airstrikes began, we have seen Saleh-aligned forces move north toward the Saudi border in response to fears of a ground incursion.
A substantial ground offensive beyond the deployment of small units of tactical air controllers or special operations forces to bolster the military strength of pro-Hadi forces remains a risky undertaking. Regardless of where an invading force entered Yemen, a sustained fight against the al-Houthis would eventually draw them into the southern part of the Sarawat Mountains, which run north to south through the country's west. The al-Houthis call this complex terrain home, making any military offensive there extremely difficult and costly. Fighting there is something Saudi Arabia and its partners will try to avoid. The best option for Riyadh is to tailor its use or threat of force to bring the al-Houthis and their allies back to the negotiating table — or at least push them back into their traditional territory in the country's north.