Tehran denies it is providing the Houthis with weapons. But Iran has repeatedly denounced a Saudi-led military coalition that has carried out airstrikes and other attacks in Yemen for more than three years since a Saudi-allied government was toppled.

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In the short term, this latest missile incursion probably won't result in any kind of direct conflict between the Middle East's two regional powers. (Though it will, undoubtedly, result in the deaths of many more Yemeni civilians from airstrikes, famine and cholera, an eminently curable disease currently ravaging the country.)

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But it will only deepen the friction between the two nations, which have been engaged in proxy conflicts for decades. It's a rivalry at the heart of unrest in Syria, Yemen, Lebanon and beyond. Here are some important ways to understand the disputes and suspicions:

Iran is the predominant Shiite power in the region, and Saudi Arabia is a Sunni powerhouse. Is this just an age-old religious war?

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It's true that Saudi Arabia and Iran are on opposite sides of an Islamic rift going back to the early decades of the faith in the 7th century. Sunnis — the majority branch of Islam — and Shiites are separated by a host of differences big and small, but it began with a dispute over Islamic leadership following the death of the prophet Muhammad. The BBC has compared the struggle between the two countries as a “version of the Thirty Years' War, in which Catholic and Protestant states battled for supremacy in the 17th century.”

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Sectarianism has certainly informed the foreign policy priorities of Iran and Saudi Arabia. Both countries have formed alliances with countries that share their version of Islam. But this isn't simply, or even primarily, a religious struggle. It's a political and economic one, a struggle for control of resources and dominance in a politically fraught region.

What kicked off this “decades-old” conflict?

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No one event spawned the tensions between Iran and Saudi Arabia. But Iran's 1979 Islamic revolution played a big role in creating the hostile environment we see today. To Saudi Arabia, the rise of the Islamic Republic posed a double threat: Its leaders were unabashedly Shiite and staunchly anti-American, opposing a close ally of the monarchies on the Arabian Peninsula.

Iran's leaders were keen to export their fervor beyond their borders. Iran's first supreme leader, Ayatollah Ruhollah Khomeini, backed Shiite militias and parties abroad. In response, Riyadh sought closer relationships with other Sunni governments. Such moves led to the formation of groups such as the six-nation Gulf Cooperation Council.

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Tensions deepened in the 1980s, when Saudi Arabia supported Iraqi dictator Saddam Hussein during the Iraq-Iran War. After the 1991 Gulf War — which significantly weakened Iraq — Saudi Arabia and Iran became “the two main regional powers,” Clement Therme, a researcher at the International Institute for Strategic Studies, told Agence France-Presse.

What are some of the more recent pressure points?

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There have been a few.

The U.S.-led invasion of Iraq in 2003 made Saudi Arabia nervous. The fall of Saddam Hussein cleared the way for the rise of Iraq's majority Shiites, who were kept on the margins by Hussein's Sunni-led regime. Iraq's new governments strongly reflected the new Shiite power.

During the Arab Spring in 2011, Saudi Arabia and Iran flexed their muscles, often backing opponents in countries with unrest.

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The 2015 nuclear deal between Iran and six world powers was staunchly opposed by Riyadh, which feared an end to Iran's international isolation. As a result, Saudi Arabia has sought closer ties with Israel, a major foe of Iran.

In 2016, things took a turn when Saudi Arabia executed a popular Shiite cleric. Iranians rioted in Tehran and attacked the Saudi Embassy, leading to the suspension of diplomatic relations.

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Tensions have escalated further with the recent Houthi missile attacks, which Riyadh claims are orchestrated by Iran.

Which side are major Middle Eastern countries on? And who's winning?

Major backers of Saudi Arabia include Egypt, Bahrain and the United Arab Emirates. Iran has close ties with Iraq, large parts of Lebanon and Syria's current leadership.

It's hard to suggest that either side is winning. But writing in the Nation, regional analyst Juan Cole explains what's behind Saudi Arabia's fear:

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Iran’s influence has gone from almost zero in the 1990s to predominant in the eastern reaches of the Middle East today. The mildly Shiite Houthi rebels staged a coup in Yemen in 2014, and deepened their control over the country the following year. That was mainly a local development, but Riyadh projected its Iranophobia on it. The pro-Iranian party-militia Hezbollah in Lebanon has dominated that country’s national unity government since 2016. Another Iranian client, the Baath regime of Bashar al-Assad, appears to have won the civil war in Syria, and the Saudi cat’s paw there, the extremist Army of Islam, has been defeated. Saudi influence in Iraq evaporated after most Sunni Arab-majority provinces seceded to join the ISIL “caliphate” in 2014, and then were conquered by the central government’s army and its Shiite militia auxiliaries. While Tehran’s relationship with the Palestinian Hamas has been roiled since 2011, the two appear to be on the mend.

What does this mean for the region?

Nothing good.

The conflict between Iran and Saudi Arabia is fueling some of the region's most intractable conflicts. In Syria, Iran has consistently backed Assad; Saudi Arabia is financially supporting rebel groups. In Yemen, Saudi Arabia has waged war against the country's Houthi rebels, complete with a blockade of the country and airstrikes. More than 10,000 civilians have died in the conflict. In Iraq, the two countries may be at odds over efforts to rebuild after Islamic State.

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And in Lebanon, experts say Saudi Arabia pressured the country's prime minister to resign in an effort to destabilize the country, where Iran's ally Hezbollah has wide influence. This could upend Lebanon's 2018 elections and disrupt the delicate balance among the country's religious groups. As the BBC explains: “Conflict in Lebanon could so easily draw in Israel in opposition to Hezbollah and this could lead to a third Israel-Lebanon war far more devastating than any of the previous encounters.”

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“The Saudi-Iranian rivalry has become the organizing principle for Mideast alliances, reminiscent of how the Cold War divided countries along U.S. and Soviet lines,” Max Abrahms, professor of political science at Northeastern University, told AFP.

Could things explode?

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It seems unlikely that Iran and Saudi Arabia will go to war, at least in the traditional sense. “A broader regional conflict remains unlikely,” Graham Griffiths, a senior analyst at consulting firm Control Risks, told AFP.

That doesn't mean that there's no reason to worry.

Michael Knights, a Lafer fellow at the Washington Institute for Near East Policy, told Vice that military tensions are as bad as he's ever seen. “All cold wars have the potential to suddenly turn hot, probably only for a moment before the leaderships 'turn off' the war,” he said. “This risk exists in the case of Saudi/GCC versus Iran.”

He continued: