Donald Trump’s decision to go to war against Iran will be remembered as a grave strategic miscalculation – one that has reshaped the region in unintended and destabilising ways. With the ceasefire now extended indefinitely, we can see more clearly how the war has undermined the US’s standing in the world and failed to achieve its core objectives: it has neither brought about regime change in Tehran, nor forced Iran to submit to American demands. Far from it.

By inflicting economic pain far beyond the region and slowing the global economy, Iran has demonstrated that its grip over the strait of Hormuz constitutes its most potent deterrent – arguably more consequential than its now defunct nuclear programme. Control of the strait will be Tehran’s most powerful source of leverage in the years ahead.

And this strategy is not confined to Hormuz. Relying on its Houthi allies in Yemen, Iran’s Islamic Revolutionary Guard Corps (IRGC) also signalled its ability to threaten the Bab al-Mandab strait at the southern tip of the Red Sea – a choke point through which roughly 8% of global trade and a significant share of the world’s energy and chemical shipments pass. The prospect of disruption at both Hormuz and Bab al-Mandab would amount to a double shock to the global economy.

Against this backdrop, it is no surprise that the US’s Gulf allies have reacted with alarm. What most unsettles Gulf rulers is the prospect of a postwar Iran wielding control over Hormuz as a permanent lever of coercion – while the US appears, at best, an unreliable guarantor of their security. The Gulf states are scrambling to hedge against this new instability by building alternative security arrangements with regional powers such as Pakistan, Egypt and Turkey, while deepening ties with Europe, China and India.

Although the US/Israel-led war has weakened Iran economically and militarily, its long-term effect may be the opposite: a more emboldened, muscular and assertive Iran. One of the war’s most significant unintended consequences is a shift in Tehran’s strategic doctrine. Rather than relying on caution and deterrence, Iran is likely to adopt a multi-front approach – escalating and targeting the wider economic and security infrastructure of its rivals and adversaries as it did in this conflict. In effect, the war has accelerated Iran’s emergence as a more assertive regional power, one with growing capacity to project influence well beyond its borders.

Within Iran, this reassessment is already under way. A new generation of officers within the IRGC appears to have drawn a stark lesson: restraint invited vulnerability. For years, the late supreme leader and his advisers had adhered to a doctrine of “strategic patience”, believing that calibrated restraint would ensure regime survival and consolidation. But the assassinations of Iran’s senior military leaders and nuclear scientists by the US and Israel and their direct attacks on Iranian territory reinforced the perception that a defensive posture no longer guaranteed security. That doctrine is now buried with Iran’s old guard.

There is already mounting evidence that the IRGC has consolidated its grip on power – directing the war effort and shaping diplomatic engagement with the US. The assassinations of Iran’s senior political and military leadership have accelerated this shift. Trump has repeatedly claimed that he achieved regime change in Tehran. In a sense, he has – but not in the way he intended.

And far from weakening the regime’s hold on power at home, the war appears to have strengthened it – at least temporarily. Despite widespread resentment and opposition toward the ruling clerics, many Iranians – like populations elsewhere under external attack – saw the destruction of civilian infrastructure not as a blow against the regime, but as an assault on the nation itself.

The result was not revolt, but a familiar wartime dynamic: a rally around the flag, reinforced by coercion and fear of state retribution. In the longer term, however, Iran will face deep structural, social and political vulnerabilities. A staggering reconstruction bill exceeding $200bn, coupled with IMF projections that inflation could surpass 70% – a historic high – will place immense strain on Iran’s economy. Unless its new rulers loosen their grip and ease their intrusion into everyday life, they are likely to encounter renewed popular resistance.

These miscalculations were not merely tactical – they reflected deeper assumptions. Trump appears not to have seriously considered worst-case scenarios such as whether Iran might retaliate by closing the strait of Hormuz. Instead, he was predisposed – temperamentally and ideologically – to accept Benjamin Netanyahu’s assurances that the war would be quick, clean and decisive.

That assumption reflected a broader pattern of strategic miscalculation and imperial hubris. Emboldened by the apparent ease with which US forces captured Venezuela’s president, Nicolás Maduro, Trump believed that Iran would prove a similarly vulnerable target. By hollowing out institutions such as the state department, the defence department and the national security council, Trump ensured that there were few restraints on his instincts, and even fewer warnings against such a consequential decision.

But there is a broader logic at work in Trump’s war of choice in Iran: this is, at its core, an imperial project. From South America to the Arctic and the Middle East, Trump has openly embraced the language of expansionism, repeatedly signalling his desire to extend American control over resource-rich territories.

Trump even treated Venezuela as a template – pointing to the seizure of its oil as proof that force could lead to similar material rewards in Iran. “To the victor belong the spoils,” Trump said, signalling his preference for a return to 19th-century imperialism. “We haven’t heard that in, I think, maybe hundreds of years,” he said.

Unlike his predecessors, who cloaked interventions in the language of international order or human rights, Trump has dispensed with such pretence. He has been unusually candid about the motivations driving his foreign policy, even describing territorial acquisition as “psychologically” important to him. What we are witnessing is not a break with US power, but its unvarnished expression.

The consequences of this approach are already visible. The geopolitical and geo-economic fallout from Trump’s debacle in Iran dwarfs that of George W Bush’s 2003 war in Iraq. By launching pre-emptive strikes while nuclear negotiations were continuing, Trump has ruptured the norms of diplomacy and set a dangerous precedent in international relations. From custodian of the postwar order, the US has become a disruptive force, aligning itself with illiberal and autocratic rulers worldwide, and one now facing a reckoning even among its closest European allies.

Future historians may see this moment as the beginning of the end of the American century, and the onset of a more uncertain and dangerous era shaped increasingly by China’s rise.

• Fawaz Gerges is professor of international relations at the London School of Economics. His most recent book is The Great Betrayal: The Struggle for Freedom and Democracy in the Middle East