On June 3, U.S. Secretary of State Marco Rubio told the U.S. Congress that the fighting that had raged for three years in Sudan was now a “proxy war” and blamed Saudi Arabia and the United Arab Emirates (UAE) for frustrating American efforts to bring the conflict to an end. Rubio was saying out loud what many observers already knew: what started as a domestic power struggle has morphed into a wider regional war, drawing in outside powers and spilling across Sudan’s borders.

The spread of the conflict in Sudan is part of a larger geopolitical transformation. For much of the past century, policymakers have treated the Horn of Africa and the Arabian Peninsula as separate strategic regions divided by the Red Sea. That distinction is now evaporating. Wars on one side of the sea increasingly alter the military calculations on the other. This entanglement is only deepening in the wake of the United States’ and Israel’s decision in February to launch a war on Iran, driving a deeper wedge between Abu Dhabi and Riyadh (which were already at odds), weakening the American-anchored security order in the region, and underscoring the importance of securing the Red Sea for shipping.

For the past decade, the UAE has been the most aggressive outside player in the Horn of Africa, establishing ports and military bases, offering large financial packages to key allies, and involving itself extensively in Sudan’s civil war. But the recent war in Iran, along with the breakdown of the Emirati-Saudi security partnership in Yemen last December, made Riyadh less confident of Washington’s protection, more anxious about the UAE’s security ties to Israel, and more intent on controlling the Red Sea’s security picture. And so as Saudi Arabia has leaned into an alignment with Egypt, Turkey, and Pakistan, it has also brought that alignment to bear to strengthen key partnerships in the Horn. It is trying to counter factions in the Horn that are backed by the UAE or Israel even as it struggles to manage threats from Iran and its allies—including the Houthis, who have seized control of Yemen’s Red Sea coast.

At the same time, the risk of conflict is increasing between the Sudanese armed forces (SAF), backed by Saudi Arabia and its allies, and Ethiopia, an ally of the emirates, as well as the chance that another direct war could erupt between Ethiopia and Eritrea. A new war between Ethiopia and Eritrea on the Red Sea would likely turn into a regional conflict that overlaps with the one in Sudan. And it would not only be aggravated by the existing Emirati-Saudi rift—it might well deepen that divide.

Some Middle Eastern governments once saw the Horn of Africa as a low-stakes theater in which they could compete for leverage at relatively little cost. But as wars in the Middle East and the Horn become inextricably linked, a single security sphere is coalescing. Unless leaders in both the Middle East and the Horn recognize how interconnected their security dilemmas have become, instability will continue to ricochet in both directions, and the overlapping conflicts and disputes will become harder to resolve.

In some respects, the growing entanglement between the Horn of Africa and the Middle East is a return to a historical norm. Over millennia, the sea connected present-day Djibouti, Eritrea, Ethiopia, and Yemen, and people, goods, and cultural trends moved regularly among them. Amharic, Ethiopia’s dominant language, and Tigrinya, widely spoken in Eritrea and northern Ethiopia, are both Semitic tongues. Christianity spread early to what is now northern Ethiopia, making it home to one of the world’s oldest Christian Orthodox traditions.

In the sixth century, an Aksumite ruler in based in modern-day Ethiopia briefly conquered parts of Yemen. Not long afterward, the Prophet Mohammed sent some of his earliest followers across the Red Sea to take refuge with a Christian Aksumite king there. Islam later spread across the sea and along Africa’s Indian Ocean coast. From the sixteenth to the twentieth century, the Ottoman Empire ruled territory on both sides of the Red Sea, and Omani rulers built an empire tying the Arabian Peninsula to much of East Africa’s coastline, eventually moving their capital from Muscat to Zanzibar. These links persisted into the modern age. When the Saudi kingdom conquered the Hejaz region on the western Arabian Peninsula in the early twentieth century, elites in the port city of Jeddah sent........

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