Iran’s shuttering of the Strait of Hormuz, and the United States’ failure to restore the free flow of maritime traffic, has put a spotlight on just how much the global economy depends on ships’ ability to pass unencumbered through key waterways—and how fragile that right is. But well before the strait’s closure, a long era of free seas was already coming to an end.

Critical maritime chokepoints around the world have become fraught with new risks. Beginning in late 2023, for instance, the Iranian-backed Houthi militia in Yemen has throttled transit through the Red Sea, forcing major shippers to reroute away from the Suez Canal and Bab el Mandeb Strait. Since at least 2022, China has mounted increasingly aggressive exercises and activities in and around the Taiwan Strait, triggering great anxiety about the vulnerability of semiconductor supply chains. Russia’s hybrid warfare in the Baltic Sea, Ukraine’s unexpected success attacking Russian vessels in the Black Sea, and even the United States’ lethal campaign against alleged drug-smuggling boats and its hot pursuit of shadow fleet tankers have all badly shaken the impression that the oceans are relatively free to navigate.

These accelerating challenges to free navigation are fragmenting the formerly open trading system. Throughout the eight decades after World War II, those engaging in commercial maritime trade—which constitutes roughly 80 percent of global trade overall—could largely afford to ignore these kinds of threats. A framework of multilateral treaties and legal norms established order in the littoral zones (coastal areas and narrow straits that channel trade) that had previously been governed by whoever could effectively control them. With unmatched military power throughout much of this period, the United States policed this relatively free zone of commerce. Now, a proliferation of challenges to free navigation is threatening the world’s open trading system. Washington still commands the strategic heights and depths, fielding the world’s most capable force of aircraft, warships, and submarines, as well as dominating space communications. But changes to trading patterns, military technology, and the economics of warfare are eroding the efficacy of U.S. military power in contested littoral zones.

Like Wile E. Coyote running off the cliff and staying aloft until he looks down, Washington has belatedly discovered that its military power cannot provide sufficient lift for its soaring global ambitions. Without the ability to control crucial waterways, straits, and other littoral zones, the United States can no longer reliably secure the economic system that has underwritten an unprecedented era of openness and prosperity. Today, both small actors and the United States’ greatest competitor, China, no longer need to challenge U.S. conventional military supremacy in these domains directly. They can do so obliquely and asymmetrically, using cheap but long-range drones and missiles or economic tools that prevent open access without firing a shot.

Even if the United States were to sink extraordinary resources into an attempt to reestablish a high degree of control over the commons, restoring the old maritime order is no longer a realistic aim. Sea changes in military technology and industrial production are undermining the basic feasibility of a truly open order. Reckoning with this structural shift is a generational challenge that must begin with making peace with the fact that the American way of war is mismatched with the contemporary world’s strategic and economic geography.

The maritime order has long been defined by a tension between states’ wish to navigate the seas freely and their temptation to control maritime space. Between the early eighteenth century and the middle of the twentieth century, these antithetical desires were reconciled by the so-called cannon-shot rule, a custom according to which a state’s authority over the sea extended only as far as its cannons could reach, a range considered to be about three nautical miles. This customary rule, recognized only by the colonial powers, permitted the strongest naval and commercial fleets to dominate the maritime commons and build global empires while assuring other states at least some measure of control over their coastal approaches. In practice, the rule afforded great advantages to a select few maritime powers, especially the British Empire. Their advanced vessels and distant naval outposts produced the first truly global trading routes and subordinated other states unable to project power beyond a narrow coastal band.

This compromise began to unwind in the early twentieth century, with the advent of modern submarine and mine warfare and the advancement of weapons technology that extended strike range from air, land, and sea. After World War II, the traditional cannon-shot rule was gradually buried under an elaborate multilateral architecture, beginning with the Geneva Conventions in 1958 and culminating in the Third United Nations Convention on the Law of the Sea (UNCLOS III) in 1982. On paper, at least, these formal rules assigned all coastal states certain geographically defined entitlements—zones of jurisdiction determined by treaty, not by naval tonnage or coastal artillery. All states enjoyed freedom of navigation, overflight, and laying of submarine cables anywhere outside a somewhat expanded, fully sovereign territorial sea of 12 nautical miles.

American might underpinned a maritime order from which most nations benefited.

Washington became this framework’s guardian—an offshore balancer operating at a comfortable geographic remove, with its power projected from highly maneuverable carriers, lurking submarines, blue-water cruisers and destroyers, and stealthy aircraft. With formal treaty rules underpinned by the U.S. military’s formidable power and ubiquitous presence, any trading vessel........

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