Yemenis hold a poster of King Salman of Saudi Arabia as they chant slogans during a rally to show support for Saudi-led airstrikes against Shiite rebels, known as Houthis, in Taiz, Yemen, March 26, 2015. (Anees Mahyoub/AP)

Saudi Arabia and Egypt are mounting a military intervention in the ongoing civil war in Yemen. They have been here before: In the 1960s, both countries intervened in the fight between North and South Yemen. However, 50 years ago, they were on opposite sides of the conflict; now they are on the same side. The switch says much about current Middle East politics and how we should understand the politics of alliances generally.

Some analysts argue that the violence in Yemen is not sectarian. That’s partially true, if one looks only within Yemen: The Houthi rebels are a heterodox Shiite group, but they have fought alongside Sunnis against the incumbent government. Locally, this is mostly a political contest for power. But if we look at the broader Middle East to see how foreign governments are aligning and intervening, it is impossible to miss the sectarian divide. Sunni governments, such as Egypt and Saudi Arabia, are backing President Abed Rabbo Mansour Hadi, who is Sunni, whereas Shiite governments, such as Iran and Iraq, as well as non-state groups, like Hezbollah, support the largely Shiite rebels.

The sectarian nature of today’s rivalries in the Middle East contrasts sharply with the last time Egypt and Saudi Arabia intervened in a Yemeni civil war. In the 1960s, Egyptian President Gamal Abdel Nasser led a pan-Arab nationalist movement that threatened the legitimacy of monarchies like Saudi Arabia. Egypt, along with Iraq and other Arab republics, supported North Yemen. Saudi Arabia and other monarchies, including Iran (which was a monarchy at the time), helped the royalists in South Yemen. Just like today, Yemen’s battle was part of the larger political contest in the Middle East – but now the central cleavage has switched from regime type to sectarian identity.

Shiite Houthi fighters have been battling a coalition of Saudi-led Sunni Muslim allies for two weeks. The conflict has raised fears that a sectarian proxy war will further destabilize the region. (Reuters)

Why has sectarianism become activated in ways that make it matter so much more than it did before? Part of the answer involves recent wars in Iraq and Syria. The U.S.-led invasion of Iraq in 2003 not only reversed the domestic balance of power between Sunnis and Shiites in Iraq, but it also unleashed insurgencies that deepened the sectarian divide across the region. The Islamic State has capitalized on and contributed to such division. In Syria, myriad rebel groups fighting President Bashar al-Assad have competed for funding and support from foreign donors. Sunni regimes have mostly funded Sunni fighters, while Iran supported Shiite fighters, including Hezbollah.

The last decade has deepened the sectarian divide, but it was politically activated much earlier, in a contest between rival narratives of legitimacy. In the 1960s and 70s, the Saudi government wanted to use pan-Islamism to counter Nasser’s pan-Arabism. When oil revenues boomed following 1973, the Gulf monarchies poured money into mosques and organizations like the World Muslim League. The influx of oil money came just at the wrong moment, when leaders and elites were looking for ways to politicize Islam. The Saudis later regretted that strategy after the Iranian revolution took pan-Islamism in a new anti-royalist direction. Paradoxically, this only drove the Saudis to burnish their own Islamic credentials more brightly, even restyling their king as the “Custodian of the Two Holy Mosques.” The rivalry between Iran and Saudi Arabia set in motion an even deeper fissure between Sunnis and Shiites.

The greater relevance of religious sectarianism today teaches us to avoid not one but two common ways of looking at Middle East politics. On one hand, many analysts and policymakers need to downgrade the importance they attribute to religion. Some analysts are arguing that the current sectarianism reflects “ancient hatreds,” thereby implying they are permanent and immutable. Yet most scholars reject the idea of ancient hatreds, arguing instead that sectarianism is a latent factor that can become politically activated by elites or circumstances. The remarkable political realignment in the parties intervening in Yemen, comparing 50 years ago to today, demonstrates that sectarian divides are not always a salient feature of politics. Indeed, the dramatic change in the course of a single lifetime illustrates the political malleability of sectarianism.

On the other hand, scholars of international relations should upgrade their estimate of the importance of religion, for alliance politics and much else. A whole generation of policymakers and scholars learned from Stephen Walt’s seminal book on the origin of alliances. Walt argued that certain things matter for the balance of threat – such as geography, offensive capabilities and threat perceptions – while others, such as ideology, do not. The book is silent on religion. Religion is certainly ideational if not ideological, which suggests that our understanding of alignments needs updating. This will not surprise specialists on religion and politics, such as Ron Hassner, Stacie Goddard and Thomas Hegghammer, but most scholars probably need to update their mental model to better account for ideational factors like religion.

Finding a middle ground between these two views of religion in politics requires nuanced understanding. In today’s Middle East, activated sectarianism affects the political cost of alliances, making them easier between co-religionists. That helps explain why Sunni-majority states are lining up against Iran, Iraq and Hezbollah over Yemen. Still, the sectarian rhetoric lies on the surface of what is a deeper and long-running conflict about regime legitimacy, what one commentator calls a battle between “Muslim monarchical rule and Muslim republicanism.” Some Sunni but republican states, like Pakistan, are resisting Saudi Arabia’s attempt to use sectarianism for regional alignments.

U.S. policymakers can see instability in Yemen and elsewhere in one of two ways. The first is as a sudden, violent upsurge of underlying sectarian hatred. The better way is to understand sectarianism as an instrument in a long-running regional contest between rival narratives of regime legitimacy. This understanding should shape foreign responses to regional events. Rare, individual security threats might call for a U.S. military response, but over the long run the situation calls for a different approach. Only local participants can resolve the contest over regime legitimacy. The United States will do damage by intervening too heavy-handedly.

Jeff Colgan is the Richard Holbrooke Assistant Professor at the Watson Institute for International Studies at Brown University. He is author of “Petro-Aggression: When Oil Causes War.”