On November 4, a U.S.-made Patriot missile intercepted an Iranian-manufactured Burkan H-2 short-range ballistic missile as its warhead plunged toward the international airport outside Riyadh, Saudi Arabia’s capital.

Though the missile was launched from Yemen, with good reason Saudi leaders called the attack an act of “aggression” by Iran. A human rights organization said the “indiscriminate” missile attack was “an apparent war crime.”

Under any circumstances, the missile attack signals that war between the Sunni Muslim kingdom of Saudi Arabia and Iran’s Shia Islamic revolutionary regime is escalating and their proxy war in Yemen will become more intense.

Iran covets Saudi oil fields, but this fight is not all about oil. Historical enmity is a factor. Both governments confront serious domestic challenges that create internal instability. Iran apparently believes that at this moment in time it is positioned to exploit Saudi domestic weaknesses — but that remains to be seen.

Since fall of the Shah of Iran in 1979, Iran and Saudi Arabia have confronted each other across the waters of the Persian Gulf. The presence of the U.S. naval forces in the region still deter overt Iranian military action in the Gulf.

Iran’s Shia regime, however, is expansionist. The ayatollahs seek to control or influence Shia Muslim communities globally, but particularly in the Middle East.

The Iranian regime concluded that the 2011 Arab Spring revolts and the U.S. military withdrawal from Iraq in December 2011 created a regional power vacuum. For different reasons and in differing guises Iranian involvement in Syria, Iraq, Yemen and Lebanon expanded, but it expanded nonetheless.

Yemen was the launch site for the November 4 SRBM because Saudi Arabia and Iran fight a “proxy war” in that miserable land.

Arab Spring chaos in Yemen presented Iran with a target of opportunity. In 2011 a revolt forced Yemen’s president Ali Abdullah Saleh to cede power in early 2012. Vice-president Abdrabbuh Mansour Hadi replaced him. In 2014, Houthi militants seized the capital, Sanaa. In 2015, they dismissed Hadi and took over Yemen’s government.

The Houthis are a political-religious movement led by the Shia Muslim Zaidi sect. Though the movement has Sunni followers and does not theologically align with Tehran’s zealots, Shia Iran began providing the Houthis with weapons, advisers and intelligence. Houthi power within Yemen increased.

If the Houthis dominate Yemen, Iran is on Saudi Arabia’s strategic rear, positioned to destabilize the House of Saud along a land frontier. The Saudis could not permit that. With the aid of the U.S., the Saudis formed a coalition to support the internationally recognized Hadi government.

So far the proxy war has killed some 9,000 Yemenis and inured 60,000. 18 million displaced people need food and medical assistance. Yemen’s total population is 28.5 million.

The Saudis conduct air strikes on Houthi targets, which is why the Houthis portray the SRBM attacks as retaliatory. The Saudis, however, are certain that the November 4 missile was fired by members of Iran’s Revolutionary Guard Corps and Lebanese Hezbollah, the Lebanese Shia militia that Iran trains and finances. Hezbollah also provides proxy fighters for Iran elsewhere in the region (Syria).

From Lebanon, Lebanese Hezbollah fires Iranian-provided missiles at targets in Israel. Iran denies involvement, while promising the eventual destruction of Israel. From Yemen, Iran can pull the same trick on the Saudis — another reason the Saudis can’t let Yemen become an Iranian base.

Does Saudi Arabia have the power to win a war with Iran in the Gulf? Not by itself. It has the assets to seed stir within Iran. Its anti-Iran coalition could extend the war beyond Yemen, but it would be an indecisive war. Without the participation of U.S. forces, toppling the ayatollah regime by military means is most unlikely.

However, the nuclear weapons clock is ticking. Iran remains committed to obtaining nuclear weapons. The Saudis have ballistic missiles and the cash to buy or build nukes. Moreover, they now have the support of a new American administration that says it won’t permit a nuclear armed Iranian dictatorship.

Austin Bay is a syndicated columnist and author.

Austin Bay