To America’s allies, President Trump’s war in Iran, now in its sixth month, seems headed for a strategic defeat.

They worry that the inability of the United States to turn military superiority into a sustainably changed Iran has displayed a vulnerability that Russia and China will welcome. In addition, Mr. Trump’s goals remain unmet, and the U.S. military is running short on key armaments that it needs for global reach.

For the moment, Iran is seemingly resolute under a more radical government, its nuclear and ballistic missile programs are degraded but not destroyed, and the Strait of Hormuz is effectively under its control. The war has left both the United States and Israel in weaker positions — and blaming each other for the mess.

Iranian missiles and drones not only restrict traffic through the strait and damage the economic infrastructure of allied energy production, but they also continue to threaten American bases across the Middle East and have killed or injured American service members.

With Mr. Trump’s aims shifting, Washington is losing credibility both militarily and diplomatically, analysts said. Its conduct in the war underscores the impression that the United States is an unpredictable instrument of global disorder, they said.

The war may evolve, but so far the result has been “a strategic loss and an embarrassment,” said Robin Niblett, a former director of Chatham House, a think tank in London. “For U.S. allies and those who rely on the U.S. for security, this is confirmation that you can’t trust the United States.”

Even if the Islamic Republic collapses, he added, “people won’t forget that their security and their future were dragged into question by an unpredictable and wavering U.S. foreign policy.”

European allies remember Mr. Trump’s vow to seize Greenland from a NATO ally and his threats to leave the alliance unless other allies met his spending goals. Middle Eastern allies see their own security undermined by this war, even if they are too committed to Washington to abandon partnerships with the United States. But they are already hedging, looking for repaired ties with Iran and better relations with Europe, China and Russia, said Ali Vaez, the Iran project director for the International Crisis Group.

Many countries disliked the global order as it was, Mr. Niblett said, but there was one. “Now America has confirmed itself as a force of disorder and everyone is drawing lessons from it, and trying to be less dependent,” he said. “No one wants to be subject to a whipsaw foreign policy that doesn’t know its limits, and that is a strategic defeat.”

Mr. Vaez cited other recent developments as evidence that the United States has lost credibility as a guarantor of allies’ security, including Mr. Trump’s abrupt change of demands last week after announcing a landmark nuclear agreement with Saudi Arabia.

“The Saudis do a nuclear deal with Trump that he walks back in 24 hours,” Mr. Vaez said. “This is the crisis of credibility of the U.S. Neither its military prowess nor its diplomatic promises can be trusted.”

Mr. Trump has argued that it is vital to prevent Iran from gaining a nuclear weapon and that it will eventually cede to superior force. Eventually, he said on Friday, he expects Iran to realize it can’t take any more punishment and will negotiate on U.S. terms.

“We’ll be hitting them. We’ll be hitting them very hard,” Mr. Trump said. “At some point, they’re going to say, ‘We just can’t take it anymore.’”

But he’s losing whatever faith he had in the Iranians, complaining they “always want to talk” but break their word after negotiations. “They’ll get weaker,” he said, “and then they peter out.”

World War II left the United States stronger at home and with more influence abroad. But since then, America’s military record has been a mixed bag. Enemies more willing to suffer the pain and expense of conflict, as in Vietnam, Iraq and Afghanistan, outlasted the United States.

This war in Iran may turn out similarly, with Washington finding a sufficient reason to cut its losses and go home.

Already, the conflict “has undermined U.S. deterrence and credibility, antagonized America’s allies and delighted its adversaries,” said Aaron David Miller, a former American negotiator and now a senior fellow at the Carnegie Endowment for International Peace. “And if the Strait of Hormuz falls under Iran’s leverage and control, it will constitute nothing less than a strategic defeat.”

A severe shortage in crucial air-defense munitions like Patriot missiles is having a secondary effect in Ukraine and showing Russia and China — as well as NATO — that the United States will be less capable for years at least to both resupply its allies and fight a lengthy war.

The war has also badly damaged the vital alliance between the United States and Israel, said François Heisbourg, a French security analyst. American officials including Vice President JD Vance have suggested that Prime Minister Benjamin Netanyahu of Israel was overly optimistic while convincing Mr. Trump that the war would be quick and decisive.

Added to already negative views of Israel and Jews more generally stemming from the long conflict with Hamas in Gaza, which some call genocide, this war has increased antisemitism in the United States and beyond, some polls show.

The most important negotiation now, he and Mr. Vaez agreed, is between Iran and Oman over control of the Strait of Hormuz, which both countries border. Iran wants sole control over the strait’s northern route and shared control over the southern route, which would give it an effective veto over traffic, Mr. Vaez said. While Oman, which Iran is convinced is influenced by Saudi Arabia and the United States, argues for a voluntary fee for passage, Iran is demanding a compulsory one.

“This may be the end of the law of the sea as we’ve known it,” said Mr. Heisbourg. Freedom of the seas has been an American vital interest from its founding, he noted, but it is now at risk.

Even if there is agreement on the strait, Iranian officials believe there is no guarantee Mr. Trump will accept it, analysts said. The Iranians remember that Mr. Trump tore up the 2015 nuclear deal and attacked Iran twice during more recent nuclear negotiations. So they are insisting on a final deal on the strait before any new memorandum of understanding can be negotiated with Washington, Mr. Vaez said, and they also want a pullback of U.S. offensive military assets.

“The Iranian calculus is simple,” he said. With its regional proxies in retreat, “they need the leverage the strait provides them to extract concessions from the U.S. and get a guarantee that an attack on Iran can’t happen again.”

Their position may be strengthened by the decision of their Yemeni ally, the Houthis, to blockade the Bab el-Mandeb strait that Saudi oil exports pass through on their way to Asia.

But even decision makers in Iran who favor further negotiations with the United States are losing confidence that Mr. Trump will be able to keep to any deal over the Strait of Hormuz or anything else, argued Esfandyar Batmanghelidj of the Bourse & Bazaar Foundation, a research institution focused on economic diplomacy.

“Iranian leaders have become intensely pessimistic about what diplomacy with the Trump administration can achieve,” he said. He noted that many of them view Mr. Trump as erratic and have a hard time seeing how they could reach a deal with the United States when it has undercut its own allies.

Washington has started a war it does not seem to know how to win or end, while Iran is mistrustful that any deal reached will be implemented. The coming weeks are likely to see a continuing cycle of ad hoc truces and escalation, Mr. Vaez said, a new form of forever war.