The controversy over weaponry is the latest, and perhaps most serious, cloud looming over the increasingly tense relationship between the United States and Turkey. If a resolution is not found, it will make dealing with every other controversy — including Syria and the imprisonment of each other’s nationals — far more difficult.

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For the Trump administration, any latent desire for compromise has run up against unyielding congressional mandates. For the government of President Recep Tayyip Erdogan, tests of political strength between Ankara and Washington have become useful distractions from a nose-diving economy.

And then there is Russia — as eager to undermine NATO unity and spread its global influence as it is to expand its weapons sales.

The roots of the controversy date back more than two decades to the U.S. commitment to the F-35, the next-generation Joint Strike Fighter. To lower the cost and ensure others would buy it, the United States formed a consortium with eight other nations — Australia, Canada, Denmark, Italy, the Netherlands, Norway, Britain and Turkey. Each was assigned a role in the plane’s manufacture under Lockheed Martin, the main contractor, and each agreed to buy some of the aircraft.

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Five major Turkish defense companies produce components including the center fuselage and cockpit displays, and Ankara became one of the largest purchasers, agreeing to buy 100 aircraft over a period of years.

Turkey’s need for the aircraft, as well as for a long-range ballistic missile air defense system, became apparent as its region grew increasingly volatile. In 2015, when a Russian attack aircraft strayed into Turkish territory during the civil war in neighboring Syria, the Turks shot it down with an F-16 fighter.

The downing came as NATO withdrew, over Turkish objections, three Patriot antimissile batteries deployed there in 2012 to protect Turkish territory from the fighting in Syria. Turkey expressed an interest in purchasing its own Patriot system but considered the U.S. response — an off-the-shelf offer without technology transfer or co-production — an insult ill-befitting a NATO ally, according to a number of Turkish officials at the time and since then.

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U.S. and Turkish officials who discussed the rocky, sensitive relationship spoke on the condition of anonymity.

Despite early signs of rapprochement between Erdogan and the new Trump administration, the fabric of the relationship was unraveling over several issues. In Syria, Turkey charged that Syrian Kurdish fighters being used as a U.S. ground force against the Islamic State were terrorists tied to Kurdish separatists in Turkey.

Erdogan implied, and many in Turkey believed, that the United States was somehow involved in an unsuccessful coup attempt the previous year. Turkey’s requests for U.S. extradition of Fethullah Gulen, a Turkish cleric who Erdogan said was the attempted coup’s mastermind, went unanswered. U.S. officials have said Turkey has never handed over hard evidence that would stand up in an extradition court.

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A U.S. pastor who had long resided in Turkey, Andrew Brunson, was arrested and charged with complicity with both the coup plotters and the Kurds, while other Turkish Americans and several employees of the U.S. Embassy there were arrested.

In this country, federal prosecutors in New York broke open a major case of Iran sanctions violations, convicting Turkish citizens and implicating a major state-owned bank for helping Iran to illegally sell oil.

By early 2017, as lackluster conversations over the Patriot missile systems continued, Russian President Vladi­mir Putin saw an opportunity and offered to sell Turkey Russia’s S-400 air defense system, considered by many military experts to be the best and most lethal in the world.

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The deal coincided with congressional passage and Trump’s signing of the Countering America’s Adversaries Through Sanctions Act, which mandates U.S. sanctions against anyone making a “significant” deal with the Russian defense industry.

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NATO has expressed concern that Turkey’s purchase of the S-400 system was incompatible with its possession of F-35s, giving Russia access to the secrets of its stealth and other technology. Turkey counters that no Russians will be involved in operating the system, which it says will use Turkish software.

Defense Secretary Jim Mattis opposed a congressional threat to include a provision in the fiscal 2019 defense budget withholding the sale of F-35s to Turkey unless the S-400 deal was canceled, saying that removing Turkey from the consortium would sharply increase costs of the aircraft and delay its production lines. As Turkish pilots began training on the planes in Texas this summer, and a deal to release Brunson fell apart, the provision passed with a caveat giving Mattis until November to explain what the fallout would be.

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A State Department spokesman, who also spoke on the condition of anonymity, said “the United States has made it unambiguously clear to Turkey at all levels that we have very serious policy, operational, and legal concerns should Turkey proceed with the acquisition of the Russian S-400 missile defense system.”

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As with so much else in the fraught relationship, the two countries have sharply different versions of what the United States has offered to meet Turkey’s air defense needs. A new Patriot proposal, the spokesman said, was presented to Turkey in March, with “updated pricing” and “prospective co-production and co-development” provisions. “Turkey would benefit from top-of-the-line technology, as well as NATO interoperability.”

“In contrast,” he said, delivery of the S-400 system could trigger sanctions for dealings with Russia, as well as “a reassessment of Turkey’s participation in the F-35 program.”

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Turkey maintains that the March memorandum delivered by Tina Kaidanow, the acting ­assistant secretary for political-military affairs, was only a vague representation, and that there has been no official offer that would make them think seriously about the Patriot system.

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Meanwhile, the Pentagon said last week that Kaidanow, a career diplomat who has been leading the Patriot discussions with Turkey, is retiring from the State Department to take a senior job in the Defense Department.

Turkey has made clear that if the F-35s are not delivered, it will look elsewhere to purchase advanced aircraft. “We are a partner in the F-35 program, and some parts are produced in Turkey,” Foreign Minister Mehmet Cavusoglu said late last month. The United States, he said, needs to “leave the language of threats” to solve the ongoing diplomatic crisis, Turkish media reported.