Any hope that Syria’s civil war could be contained within that country ended over the weekend. The major military force in neighboring Lebanon, Hezbollah, officially declared it had joined the fight. In fact, its declared entry triggered violent sectarian clashes within Lebanon.

The question now is whether the Syrian conflict, which began with peaceful pro-democracy protests in 2011, could erupt into a religious war between branches of Islam, pitting forces backed by Shiite Iran, such as Hezbollah, against militias backed by Sunni-run states like Saudi Arabia or even Turkey.

The prospect of a “holy war” in the Middle East adds urgency to joint calls by Russia and the United States for an international conference on Syria. The risks of an intra-Muslim conflict engulfing the region are too great to ignore this plea for peace. One need only look at the ongoing violence in post-Hussein Iraq to see how intractable a Sunni-Shiite conflict over Syria might be.

While the Syria war had already attracted foreign Sunni jihadists and special Iranian forces on either side, Hezbollah’s entry brings heightened concern. Its leader, Sayed Hassan Nasrallah, says his Shiite forces are in Syria to kill “takfiris,” or Islamists who urge Sunni Muslims to kill anyone they consider an infidel. He also wants to defend a key corridor across Syria long used to ship Iranian military equipment to Hezbollah (“Party of God”).

A pure war over religious doctrine is rare in history, and that may hold true in this case. Much of the struggle in Syria is a geopolitical contest between Iran and Saudi Arabia for influence in the region. This competition began after the Iranian revolution in 1979 and accelerated in 2003 when Iraq shifted from being led by a Sunni-led regime to one that is Shiite-dominated and Iran-leaning.

Sunni-led Turkey, too, has increasingly thrown its weight behind the Sunni-led Gulf states, especially in its agreement to host a NATO early- warning antimissile system aimed at Iran. Turkey also opposes the Syrian regime of President Bashar al-Assad, whose minority Alawite community is a branch of Shiism.

In most Middle East struggles, a religious conflict often emerges because rulers tend not to trust people who practice a differing branch of Islam. At times like this, Muslims must remember the religious values they share and their common hope for peace rather than killing each other in the name of Sunni or Shiite interpretations of Islam. The Arab Spring was, in part, based on a hope for tolerant, diverse, and democratic societies that respect religious differences.

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That spirit of the Arab Spring still exists within the political opposition to the Assad regime, even though jihadist fighters dominate most of the antiregime battles within Syria. Within Lebanon, too, many people oppose Hezbollah’s move into Syria, preferring to avoid the sectarian strife that ripped their country apart from 1975 to 1990. Hezbollah may pretend that its combat role in Syria is really a struggle against Israel, but few people will buy that.

As more outside forces intervene in Syria, the more the war becomes a “clash within a civilization,” or intra-Muslim struggle. But as more countries recognize this dangerous course, the more they must back talks to end it. Holy wars are never holy.