Militants loyal to Yemeni President Abed Rabbo Mansour Hadi patrol in the southern port city of Aden on Monday, March 23, 2015. Reuters/Nabeel Quaiti

The suicide terrorist attacks in Sanaa on Friday that targeted Shiites and killed at least 137 people, combined with the rapidly escalating sectarian fighting, are threatening to plunge Yemen into a civil war, similar to what’s happening in Libya, Syria and Iraq. But the splits in Yemeni society are originally tribal rather than along the Sunni-Shiite sectarian line.

Yemen is now split between the Iran-backed Houthi rebels that seized the capital last year, who are Shiites, and an elected government led by a Sunni. The country also hosts the most effective branch of al Qaeda, is vulnerable to infiltration by the Islamic State group and sits on a busy international trade route on the doorstep of Saudi Arabia.

Roughly 75 percent of Yemenis are members of various tribes, whose allegiance is based on protection and economic benefits, not religion. However, the rise of the Shiite Houthis has given Sunni jihadis such as al Qaeda in the Arabian Peninsula (AQAP) and the Islamic State group (ISIS or ISIL) room to boost the sectarian conflicts in which they thrive.

“The Shiite-Sunni clash was transplanted from Iraq and Syria into Yemen,” said Peter Knoope, an associate fellow at International Center for Counter-Terrorism, an independent think tank based in The Hague. “The potential was always there, but outside forces … (have) inflamed the actual outburst over the last months. Yemen has entered a new phase and has become part of a bigger agenda in the region.”

Saudi Arabia, which backed President Abed Rabbo Mansour Hadi’s government, halted its financial support when Hadi was essentially deposed by the Houthis. That left free rein to Iran, the Shiite powerhouse supporting the Houthis, and the Houthis have been advancing. Their progress into areas of the country where their tribe has no presence, using the excuse of defeating al Qaeda, has created resentment onto which external forces have latched.

Yemen’s tribes are bound by a shared culture rather than religious ties, and the country’s history of political chaos has given tribal leaders a great degree of autonomy, which they now believe the Houthis are threatening.

“The local populations are not fighting because the Houthis are Shia or because they’re Sunni,” said Katherine Zimmerman, a research fellow at the American Enterprise Institute (AEI) and the lead analyst on al Qaeda for AEI’s Critical Threats Project. “They’re fighting because the Houthis are trying to come down and establish a presence where the Houthis have no legitimacy.”

Tribal-heavy areas also tend to be the oil-rich regions of Yemen. The Houthis are now threatening to enter the Marib governorate, the source of much of the country's oil and gas production and a place where the land is owned by local tribes.

“The tribes are uniting against the Houthis, because they simply don't want to see someone else come in and take resources from their land,” Zimmerman said. “These people don’t see the Houthis as being representative of them; they’re not the central government. It’s like a foreigner is coming into their land.”

That attitude has been cemented by decades of government weakness and neglect. “The strong presence of tribes in Yemen is due to the corruption and weakness of the state institutions there,” Nadwa al-Dawsari wrote in “Tribal Governance and Stability in Yemen,” published by the Carnegie Endowment.

Each Yemeni tribe is led by a sheikh, appointed because of his ability to manage conflict resolution, provide public services and especially protect the group’s financial interests. Tribes abide by their set of laws and have historically been “anti-ideological and are fearful of the appeal of radical groups, such as al Qaeda,” according to al-Dawsari.

But in recent months certain tribes have aligned themselves with AQAP fighters, as the two now share an enemy in the Houthis. AQAP is able to provide tribal leaders with protection and resources that they are unable to receive from Hadi’s government, Zimmerman said.

Saudi Arabia has been funding Yemen for decades, and the kingdom’s financial support only increased after Hadi took power in 2012. The Saudi monarchy supported his government with nearly $4 billion, in addition to money pledged by the Sunni states of the Gulf Cooperation Council. Since Hadi’s resignation, Saudi Arabia suspended funding and shut down its embassy. The kingdom’s exit from Yemen was soon emulated by several Western countries, including the U.S., which pulled out its last remaining special forces on Monday.

The Houthis, who are now in charge, are seeking to substitute new actors for their support. Besides Iran, Houthis are open to involvement from a variety of countries. Last month, Houthi representatives met separately with Russian and Chinese delegations to discuss economic alliances.

“America is not everything,” said Hussain al-Bukhaiti, a pro-Houthi translator and journalist in Sanaa with close family connections to the group. “If America and the Gulf states want to boycott you, there are other countries.”

Whoever will be Yemen’s next financial backer will find a very troubled situation. Yemen is still the poorest country in the Arabian Peninsula, with a GDP per capita less than one-tenth of neighboring Saudi Arabia. Roughly half of its population is below the poverty line. Almost 50 percent of Yemenis don’t have access to clean water, and slightly less than a third don’t have basic health care.

Yemen ranked eighth out of 178 in the Fund for Peace’s 2014 Fragile State Index, which is based on several factors, including sectarian violence, poverty, state security, public services and the government’s legitimacy. It scored worse than Iraq, Syria and Libya.