Yemen's Houthi leader Abdel-Malik al-Houthi on Sunday vowed to send his fighters to the southern port city of Aden to rout both Al-Qaeda and forces loyal to President Abed-Rabbo Mansour Hadi. The threat came just hours after Houthi fighters seized the country's third-largest city Taiz — a move that inches the country dangerously close to a multi-front civil war drawing in regional powers Saudi Arabia and Iran. Branding the advance by the predominantly Shia Houthi movement an "Iranian aggression," Saudi Arabia's foreign minister Prince Saud al-Faisal on Monday vowed to "take necessary measures" in defense of Hadi's government, which had reached out to the Saudi-led Gulf Cooperation Council (GCC) requesting urgent military assistance. Some analysts saw the growing turmoil as opening the way for a potential comeback by former president Ali Abdullah Saleh, ousted during a national uprising in 2012. Saleh retains considerable support in Yemen's security forces, and has been accused by Hadi of colluding with the Houthis to obstruct efforts to reform the government and armed services. The situation is complicated by the regional power struggle often waged on sectarian lines, and by the fact that Yemen is the base of Al-Qaeda in the Arabian Peninsula (AQAP), of one of the network's most successful remaining franchises. Yemen has faced an unprecedented upsurge of violence over the past week, including the deadly bombings of two mosques in the capital Sanaa used by Zaydis, a Shia sect linked to the Houthis, which left 137 people dead. The Houthi movement blamed the attacks on allies of the president, and called Saturday for a general mobilization against Hadi, who fled Sanaa last month after Houthi rebels took over the capital and placed him under house arrest. Hadi established a temporary capital in Aden, Yemen's main economic hub located in the country's formerly independent south where he remains popular. But Saleh boasted last week that he would corner Hadi in Aden. On Thursday, forces loyal to the former strongman stormed Aden's international airport and sent warplanes to bomb Hadi's palace. Although troops loyal to Hadi fought off that attack, he now finds himself embattled by both the Houthis, who control at least nine of Yemen's 21 provinces, and forces loyal to Saleh. The dramatic events of the past week left many observers, and the United Nations Security Council, fearing that Yemen had descended into civil war. "It will be a slow march towards the south," said Abdul Ghani Iryani, a Yemen-based political analyst. "We will be able to make sense of it in a few days when the level of resistance becomes clear."

Watershed moment

Last week's mosque blasts, one of Yemen’s deadliest sectarian attacks, were seen by many as a watershed moment in the protracted power struggle. "It’s a new threshold of violence and marks the tip of the iceberg," said Bernard Haykel, Professor of Near Eastern Studies at Princeton University and a leading scholar on the Middle East. "It's the beginning of much worse things to come.” The Islamic State in Iraq and the Levant (ISIL) claimed responsibility for the mosque bombings. If true — and the U.S. among others expressed skepticism — the attacks would be the first by the group in Yemen, adding a new layer of complexity to the country's turmoil and amplifying its sectarian dimension. "The Houthi narrative is sectarian, accusing their opponents of being AQAP or ISIS," Iryani said. "The other side finds itself in one trench with AQAP and militant Sunni groups. As blood flows, the attractiveness of sectarian hateful narrative will increase." Some Yemenis expressed doubts that ISIL was behind Friday's attacks, believing the more established AQAP was responsible. The Al-Qaeda franchise, with strongholds in the vast and lawless east, has exploited Yemen’s turmoil to step up sectarian attacks on Houthis. But AQAP on Friday denied carrying out the mosque attacks. Haykel speculated that the attackers could have come from a group that had broken from AQAP to join ISIL. Regardless of the exact author, the sectarian impact will be the same. "Whether its AQAP or ISIS, they are parts of the same political force – they comprise militant Islamist groups, where membership is interchangeable," Iryani said. Those organizations share the same goal, according to Adam Baron, a Yemen expert at the European Council on Foreign Relations. “They want to stoke sectarian tensions to a dangerous new level in Yemen." The Houthis' move to confront Hadi in the south suggests those groups intent on sparking sectarian strife may have succeeded. The Houthis accuse Hadi of colluding with AQAP, and some analysts note that Hadi's "popular committees," established several years ago to confront AQAP, include former members of the organization, some of whom still maintain links with it, said Iryani. But he emphasized that Hadi himself is unlikely to have direct ties with AQAP.

Hadi cornered

Should Houthi fighters backed by security forces loyal to Saleh successfully enter Aden, that would likely spell the demise of Hadi's leadership, say experts. That prospect appears to have prompted the Saudi warning. Hadi and his loyalists dominate Aden, but two army units in the city are loyal to Saleh, as is a force of 3,000 special forces under Brig. Gen. Abdul-Hafez al-Saqqaf. It was Saqqaf's forces that stormed the airport early Thursday, sparking battles with pro-Hadi forces. Saqqaf fled to Taiz, a major city he helped the Houthi forces capture on Saturday. "We do not have an air force, which will make it difficult to defend Aden,” said Summer Nasser, an activist and member of the Yemeni American Coalition for Change currently living in Aden. “Tribesmen and Hadi's militias are in high presence in Aden to protect the city from Houthi control." Baron agrees. "I don't see how Hadi could win alone. He's at risk of being trapped in Aden." Following the fall of Taiz, Yemen’s legal government — Hadi's — on Sunday urged the U.N. and GCC to impose a no-fly zone over parts of the country, reported the pan-Arab newspaper Asharq Al-Awsat. Yemen’s Foreign Minister Riyadh Yassin warned that without military intervention by the GCC, the country “is heading towards a civil war". If Aden falls, analysts warned, that could signal Saleh’s formal return to politics. "The Houthis are overextended militarily and politically," said Casey L. Coombs, a freelance journalist based in Sanaa. "Saleh’s the one with the money, a professional army and the experience with this very type of situation." Propaganda material supporting a Saleh return is starting to eclipse Houthi material, he added. There have also been a number of demonstrations in Sanaa over the past week expressing support for Saleh's eldest son Ahmed, according to Haykal Bafana, an attorney based in the capital. Bafana said that Houthi politics were beginning "to fade," leaving Saleh and his supporters with an opportunity to reclaim control of the country. "There is friction within the Houthi movement and their allies have almost completely disintegrated since they first rose to power in September. The alliance between Saleh and the Houthis was never a strong one to begin with," Bafana told Al Jazeera. "I think the only unified military that can still function are the Republican Guard." Ahmed Ali Saleh – Yemen’s current ambassador to the United Arab Emirates – is the former head of the national security forces that were known as the Republican Guard under Saleh. He was transferred to his new post by Hadi's transitional administration, which sought to purge the government and military of Saleh’s cronies. "Saleh's definitely angling to put his boy in the palace while Hadi mainly insists on the impossible: new dialogue at GCC headquarters in Riyadh," Coombs said. For the moment, the GCC supports Hadi as the legitimate leader of Yemen, but Baron says that could all change should Saleh, with the help of the Houthis, overtake Aden. "GCC states are backing Hadi for now but the question is for how long? Anything is possible. Saleh can spin it as him being able to keep a lid on the GCC adversaries including the Houthis, who the GCC accuses of being agents of Iran," said Baron. Coombs agrees. "Stability, reliability, are some if not the most important factors in GCC choice for Yemen’s next leader, at the end of the day," he said. For his part, Iryani is skeptical that Saleh will succeed in his ambitions, at least in the short term. "There are voices in the GCC calling for cooperation with Saleh, but I don't think that will work. That is Saleh's plan, but this will not happen until everyone else runs out of options." He added, "More likely, the final scene in this tragedy will be a showdown between Saleh and the Houthis."

Hadi's biggest asset

Experts say there is one important figure who could determine which way the conflict sways — Defense Minister Maj. Gen. Mahmoud al-Subaihi, a Hadi loyalist. "I think he's the key figure. He fled from Sanaa to Aden. He's one of the more respected people in the country and commands a decent amount of respect in the Yemeni military and has troops loyal to him," Baron said. Subaihi is considered one of Hadi's strongest allies, explained Baron. It was Subaihi's troops who helped repel Thursday's attacks against Hadi's palace. During more than four hours of fighting, a convoy of tanks led by Subaihi arrived from downtown Aden to reinforce the airport's defenders. After warding off the assault, his troops surrounded the adjacent base of the pro-Saleh police commandos and pounded it with artillery before storming it, officials said. "Al-Subaihi has the potential to play a key role in shaping how things move forward," Baron said. Following the fall of Taiz, Subaihi successfully unified military units in and around Aden, creating a unified front in case of a Houthi onslaught. "He is of course a central figure in this, as the only credible military figure in Hadi's camp — credible in terms of his ability to command forces," Iryani said.

Yemen's social fabric frays