Over the last half millennium, societies around the globe competed to divide up the world’s land. Nearly every square inch of soil outside Antarctica is now owned and tucked within national borders. But the oceans have largely been spared that fate. There were times throughout history when countries tried to gate and regulate the seas, but they were too vast to be controlled for long periods. During the Cold War, as technology made it far easier to imagine partitioning the world’s waterways and seabeds, great powers avoided an unrestrained race to grab the oceans. Most governments agreed to deal with them through a patchwork of treaties and multilateral bodies that sought to guarantee freedom of navigation in international waters and assign coastal seabeds to adjacent littoral states.

That order is now collapsing. The Houthis are disrupting shipping in the Bab el Mandeb Strait, the gateway to the Red Sea. Iran and Oman are planning to charge ships fees for using the Strait of Hormuz, a chokepoint for global energy supplies and fertilizers. In July, U.S. President Donald Trump even floated the idea that the United States could impose its own toll on Hormuz, only to later drop the plan.

Other governments, too, are jockeying to claim international waters and marine resources for themselves. Russia is trying to assert control over vast swaths of Arctic waters and seabeds, China is building artificial islands in the South China Sea, the United States has expanded its claims to seabeds beyond its exclusive economic zone, and many countries are laying the groundwork to mine the ocean floor for critical minerals.

The institutions that have kept the oceans free and open are failing, and great powers have, in key areas, stopped abiding by the rules that have held for decades. States are already making moves to control the seas. If governments rush to claim the world’s waters, as they did its land, they will make global commerce more difficult and encourage a new era of seaborne conflict. A world in which the oceans are carved up will invariably be poorer and more dangerous.

FOR WHOM THE STRAIT TOLLS

Governments around the globe are taking note of the success of the maritime blockades in the Middle East. Just as Iran announced that it intended to introduce a toll in Hormuz, Indonesia’s finance minister suggested that Indonesia might follow suit in the Strait of Malacca, a critical shipping lane in Southeast Asia that carries roughly 29 percent of global maritime oil flows. (He eventually walked back the idea.)

The closures of the Strait of Hormuz and the Bab el Mandeb Strait are just the latest in a........

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