The Saudi-led coalition has waged a dynamic campaign, but all actions so far constitute a prelude to the tougher fighting ahead should the coalition launch an assault on Sana and the Houthi heartland.

March 26 marks one year since the Saudi-led Gulf coalition began military operations in Yemen to defend the embattled government of President Abdu Rabu Mansour Hadi, who had been forced to flee the country in the face of offensives by Houthi militias and their allies. The anniversary provides a useful moment to examine the coalition's successes and failures in each phase of the war. Part 1 discusses the ground campaign; part 2 examines the air campaign, and part 3 discusses the naval blockade effort.

Shortly after United Arab Emirates (UAE) and Saudi special forces were first observed in combat in Aden, the Gulf coalition campaign secured the port city as a base for President Hadi's government throughout May and June 2015. It then transitioned from July onward into a broader offensive that saw UAE-led armored columns strike northward to al-Anad Air Base -- the former U.S.-Yemen counterterrorism facility located twenty-five kilometers north of Aden -- and Taizz, Yemen's third largest city, and also east to Zinjibar, the provincial capital of Abyan province.

AUTUMN OFFENSIVES

Last fall, the Gulf coalition opened up multiple fronts in the war against the Houthis and their allies in the Afaash clan of the Sanhan tribe, which backs the ousted former president Ali Abdullah Saleh. These fronts include:

The eastern front in Marib. In mid-September, a two-brigade-strong Gulf coalition battle group including a UAE armored battalion plus Bahraini, Saudi, Qatari, and Egyptian mechanized forces launched from a new forward operating base (FOB) at the Safir refinery airstrip in Marib province toward Marib city. The strike force advanced 50 kilometers in two weeks, taking Marib city and the Marib dam, 110 kilometers east of Sana, before bogging down against increasing Houthi and Saleh-loyalist Yemeni army resistance in the rocky desert and mountain areas between Marib city and Sana.

The Bab al-Mandab offensive. In early October, a second Gulf coalition strike force launched a powerful long-range advance from Aden toward the Bab al-Mandab Strait. The Gulf Cooperation Council southern battle group pushing toward Bab al-Mandab included a UAE battery of self-propelled howitzers, a battalion of mine-resistant ambush-protected vehicles (MRAPs), and a mixed task force of company-sized units, including a UAE Leclerc tank company and Saudi and Bahraini mechanized units, advancing under heavy air and attack-helicopter support. The strike force met with little resistance during its rapid 160-kilometer advance from Aden to the Bab al-Mandab. By the second week of October, Houthi and Yemeni army forces holding the strait had pulled out. Having occupied the strait, the UAE-led strike force pushed toward the port of Mokha twenty-five kilometers to the north. Mop-up operations continued into the fall, with Saudi and Egyptian airstrikes and naval gunfire supporting naval and helicopter landings to clear the Red Sea islands.

SHARING COMBAT OPERATIONS

As the advances north from Bab al-Mandab and west from Marib petered out in mid-October, the war became more static. Hadi loyalist forces and tribal militias sought to break the Houthi siege of Taizz but became locked in seesaw fighting in Taizz, Dhale, and al-Bayda provinces. The Gulf coalition used this period to transition from a high-visibility ground combat role to training and supporting Yemeni forces on the battlefield, as follows:

Development of a close air-support base in Yemen. By mid-October, the Gulf coalition had developed al-Anad Air Base into a major FOB, a close air-support air base, and a training base for the new Yemen National Army (YNA). The base was garrisoned by the Saudi 6th Airborne Brigade and the 64th Special Forces Brigade along with UAE ground forces. Also deployed to al-Anad were a Saudi AH-64D Apache squadron and UAE Bell 407 light attack helicopter. In concert, the UAE air force began a program to form a new Yemeni air corps, training Yemeni pilots on UAE-provided AT-802 light strike aircraft operating from al-Anad. By late October, the Yemeni aircraft had begun providing close air support to Hadi loyalist forces in Taizz and al-Bayda.

Large-scale train-and-equip mission. YNA forces combining Hadi loyalist army units with "popular resistance committee" militias and Somali and Eritrean recruits began training at al-Anad and the nearby Labouza military base in October. By March 2016, eight Gulf coalition-trained and equipped YNA brigades were operational: the "Salman Decisiveness" Brigade formed in Aden in September 2015, plus the 1st, 2nd, 3rd, 4th, 19th, and 22nd Infantry Brigades, and the 14th Armored Brigade, along with unspecified special forces and naval infantry units.

Eritrean base and forces. In May 2015, Saudi Arabia and the UAE concluded a new military security partnership agreement with Eritrea allowing the Gulf coalition to use its land, airspace, and territorial waters for Yemen operations. The agreement also included a thirty-year lease for the port of Assab, on Eritrea's coast, as a UAE naval logistics hub. Since September, the UAE has used Assab as a launchpad for amphibious operations against Yemen's Red Sea islands and in November began flying strike sorties over Yemen from Asmara International Airport -- strikes that followed an agreement with the Eritrean government to refurbish the airport. Four hundred Eritrean troops were also contracted to serve embedded with the UAE armed forces in Yemen.

Sudanese hold force for Aden. In mid-October, UAE landing craft transported two Sudanese battalions equipped with BTR-70 armored personnel carriers from Assab to Aden. The Sudanese units assumed responsibility for security in Aden as UAE forces pulled back to their bases. A third battalion arrived on November 7 to provide security at al-Anad Air Base, bringing the country's Sudanese presence to a full two-thousand-man brigade.

THE TROUBLED NORTHERN FRONT

The least successful aspect of the Gulf coalition has consistently been the fighting along the Saudi-Yemen border. Throughout the war, Houthi forces and former Yemeni army units loyal to Saleh have intensified their cross-border attacks into the southern Saudi areas of Asir, Jizan, and Najran. The scale of the problem for Saudi Arabia is alarming: what began in late summer 2015 as convoy ambushes and attacks on small border forts has developed into large-scale incursions. Whereas individual border outposts were briefly overrun in August, now parts of depopulated Saudi border towns are being occupied and large Saudi border guard headquarters held for long enough to be demolished.

The tactical challenge posed by the Houthi and Saleh-loyalist forces recalls Lebanese Hezbollah's border wars against Israel. In the last six months, missile teams equipped with Iranian-supplied 9M113 Konkurs, 9M133 Kornet-E, and Toophan missiles have knocked out around sixty Saudi tanks and other vehicles, plus a dozen border outposts and watchtowers, allowing squad- and platoon-sized Houthi and pro-Saleh infantry groups to raid continuously across the border. Saleh-loyalist artillery units have bombarded Saudi bases and rear-area garrisons with BM-27 Uragan 220-mm rocketfire and isolated Saudi bases by dropping antipersonnel mines on their reinforcement routes. Meanwhile, facing heavy attrition from air attacks, Houthi infiltrators have sought to use shoulder-launched man-portable air-defense systems (MANPADs) to keep Saudi attack helicopters at bay.

RENEWED PUSH ON SANA?

After a reset period, the Gulf coalition looks to be reinvigorating its offensive plans in northern Yemen through the following actions:

Pincer movement on the Red Sea coast. In early January 2016, Saudi marines and YNA troops landed at the port of Midi near the Saudi border in Yemen's Hajjah province. By February, they had broken out of their beachhead and were pushing inland toward Hajjah city. Gulf coalition forces also began a northward drive from the Bab al-Mandab toward the port of Mokha to isolate Taizz from the west.

Relief of Taizz. The Red Sea coast offensives were followed by a major offensive to relieve the Houthi siege of Taizz. The Hadi loyalist 22nd Brigade -- trained at al-Anad Air Base -- and local Islah Party and tribal popular resistance committee militias punched through the Houthi cordon from the west and relieved the city.

Renewed threat to attack Sana. From December 2015 onward, the Gulf coalition inched toward Sana using Saudi-trained and equipped YNA and local tribal militias resupplied by airdrops. On December 20, Saudi paratroops were airdropped into the Nihm district in eastern Sana province, enabling a leapfrog advance by Hadi loyalist forces and local militias along the northern Sana-Marib road link. The Republican Guard base at Beit Dahrah, twenty kilometers from Sana's outskirts, fell on December 21, and on February 24 militias supported by Saudi and Emirati airstrikes defeated two Saleh-loyalist Yemeni army brigades and captured the base of the Saleh-loyalist 312th Armored Brigade at Fardhat Nihm after a two-week siege. On February 24, Hadi loyalist forces took Camp Arqub, a large military base forty kilometers southeast of Sana, surrounding the capital from east.

Arrival of new offensive forces. Recent indicators suggest that the Gulf coalition forces are preparing to restart the offensive, undertaking simultaneous offensive operations in several provinces. Notably, a UAE mechanized battalion task force accompanied by Leclerc tanks recently arrived in Aden city.

ASSESSING THE CAMPAIGN

As the military analyst Tom Cooper has noted, the Gulf coalition underestimated many factors working against it: the resilience of Yemeni army loyalty to Saleh and the Houthis, the effects of terrain, the Houthis' fighting capabilities, and the complications posed by al-Qaeda in the Arabian Peninsula and Islamic State operations in southern Yemen's liberated zones. Faced with these challenges, the Gulf coalition froze its major offensive plans and expanded its available forces through a major train-and-equip effort aided by expansion-of-force contributions by coalition members. Despite holding considerable numerical superiority, Houthi and Yemeni army units have been forced to slowly retreat into Sana and northern Yemen. The ground campaign so far has been a qualified success: it restored the Hadi government to Aden, denied Taizz to the Houthis, and placed pressure on both Sana and the ports on which the Houthis rely for resupply.

The question is now whether the Gulf coalition's successes at the peripheries of Houthi control can be replicated in its heartland in Sana city and Saada province. Many indications suggest that the Houthis and Saleh forces have reserved their best units and capabilities in these heartland areas and that the campaign so far has largely been a prelude, a delaying action, anticipating the main battle in northern Yemen. This points to the importance of internationally mediated conflict termination before the Gulf coalition begins an assault on Sana or the Houthi home areas in Saada, which would be very difficult militarily and could result in protracted conflict with heavy civilian casualties.

Alexandre Mello is lead security analyst at the energy advisory service Horizon Client Access.

Michael Knights is a Lafer Fellow with The Washington Institute.