For eight decades since the end of World War Two, the United States has stood as the world's dominant power, underpinned by unmatched military, economic and diplomatic strength. Yet a leading global security expert argues that President Donald Trump has hastened the end of that era — and that while a number of his policies have contributed to American decline, none has exposed the country's diminished capacity more starkly than the spiraling war with Iran. The case matters because it cuts to a long-running question in U.S. strategy: whether American forces, ships and stockpiles can cover commitments in the Middle East, Europe and the Indo-Pacific all at once.

Phillips P. O'Brien, a professor of strategic studies at the University of St Andrews in Scotland, wrote on Wednesday in The Atlantic that Trump has pushed the end of American dominance into "plain sight."

"In the past six months," writes O'Brien, "America's power — long exemplified by the world's best warfighting equipment and most highly trained military personnel, a global network of bases and logistics systems, and military and diplomatic partnerships with all corners of the world — has proved insufficient to defeat Iran, a repressive, economically stagnant regime armed with cheap missiles and drones."

O'Brien stresses that Trump did not erode U.S. capabilities single-handedly, but argues that his "war with Iran has revealed just how the underlying dynamics of global power have shifted — and just how much American might has decayed."

The evidence, he writes, is substantial. Iran no longer fears being overwhelmed militarily, and its inexpensive drones and missiles have destroyed military bases and air defense installations across the region. The strikes have disrupted supply chains, reduced the U.S. military presence and produced a major drop in morale. The United States has now outright abandoned key military sites across the Middle East. When one service member asked Admiral Daryl Caudle about retrieving personal items from a major base in Bahrain, Caudle admitted, "We're not getting back in there anytime soon. I just want to be completely honest with that." Taken together, the setbacks have prompted the U.S. military to "reconsider" its presence in the Gulf entirely. Whether that reconsideration hardens into a formal, lasting reshaping of the American footprint in the region is one of the open questions the essay leaves behind.

O'Brien describes these problems as a cascading effect. "To maintain its military effort against Iran, the United States has had to strip the western Pacific of vital equipment," he writes. "The Trump administration finally relieved the Abraham Lincoln last month; taking its place in the Arabian Sea was USS George Washington, which is normally based in Japan. Its normal task is to discourage military action by China — a deterrence that is now greatly weakened." The carrier swap illustrates the arithmetic underneath his argument: hulls, aircraft and munitions exist in finite numbers, so every asset pulled toward the Gulf thins the layer of deterrence somewhere else. The war has also depleted stockpiles of weapons that are to deterring China and Russia, while declining U.S. manufacturing capacity has made it impossible to catch back up.

Allies, once a source of amplification for American power, no longer serve that role, according to O'Brien. Trump "perceives governments that have followed American leadership for decades as free riders, annoyances, even hindrances," and has done everything he can to disrupt these vital diplomatic ties. He has withdrawn military support for Europe, launched a trade war with Canada and brushed off South Korea. As a result, all of those allies are now looking for security and economic solutions that exclude the United States. In his framing, a dispute Washington has often cast as burden-sharing has become, in allied capitals, a search for alternatives — and whether those efforts harden into durable arrangements is likely to be read as a test of his thesis.

Nor, O'Brien suggests, is there likely to be a way back. "Even if the United States tries to revert to more of its earlier way of doing business, recent events suggest that its global reach will decrease," he writes.

American decline has gone from being almost unthinkable to starkly apparent. "As recently as February, few experts would have predicted that the United States — a global colossus for more than 80 years — would end up so hamstrung by Iran," O'Brien observes. Facing this disaster, he says, it is impossible to avoid drawing an obvious conclusion: "The American age has come to an end."

This article originally appeared on Alternet.