The increasingly tense relationship between the US and Turkey has deteriorated further, with each country abruptly imposing sweeping new travel restrictions against the other’s citizens.

The move marks an unsettling new milestone in the slow but steady decline in the US-Turkey relationship — a crucial alliance for both countries — and some experts are saying that Washington and Ankara are now facing a historic crisis that will be very difficult to resolve.

“I think the whole thing could go off its wheels,” Soner Cagaptay, an expert on Turkey at the Washington Institute for Near East Policy, told the Washington Post. “There is a very, very deep trust deficit in bilateral ties.”

The new fight began Sunday, when the US announced it was suspending most visa services at its diplomatic facilities across Turkey. The new restrictions mean that Turkish tourists, students, diplomats, journalists, and businesspeople will be barred from getting the paperwork needed to visit the US.

Just a few hours later, Turkey announced a measure that mirrored the US’s ban almost exactly.

The US and Turkey are NATO allies, but in the past year they’ve clashed over many issues, including Turkish President Recep Tayyip Erdoğan’s jailing of dozens of US citizens for their alleged ties to a failed coup attempt against him last year and the US’s refusal to extradite Fethullah Gülen, a Pennsylvania-based Turkish cleric whom Erdoğan accuses of orchestrating the coup attempt.

In announcing the move, the US diplomatic mission in Turkey said “recent events” have forced the US to “reassess the commitment” of Turkey to the safety of its staff.

Analysts say that’s a reference to Turkey’s arrest of a US consulate employee in Istanbul last week. Turkey claimed the man — a Turkish citizen — was linked to Gülen’s movement, but the US Embassy said at the time of the arrest that the accusations were “wholly without merit.”

Turkey responded in kind hours later, appearing to almost copy and paste the language of the US’s visa announcement into its own.

The Turkish suspension announcement included a reference to a freeze on electronic visas and visas bought at the border, which is the way most tourists and short-term visitors enter Turkey.

Analysts say the visa fight doesn’t bode well for either country. Turkey’s tourism industry will likely take a substantial blow — tens of thousands of US nationals have visited Turkey annually in recent years. More than 37,000 US nationals traveled to Turkey in 2016. (That was a big drop from the 88,000 US visitors in 2015, a change that can be attributed to the coup attempt and security crackdown in Turkey last year.)

And many of the Turks who will now be locked out of future travel to the US are very likely to be sympathetic to the US and critical of Erdoğan’s regime, according to Nick Danforth, a senior policy analyst focused on Turkey at the Bipartisan Policy Center.

Influential critics such as politically disenchanted scholars or businesspeople are important sources of information for the US about how Turkey’s increasingly authoritarian government is evolving — and how to respond to it.

It all seems to be a bit much for one Turkish arrest last week. But the arrest is really only the latest clash among many that have pushed the US and Turkish governments to a point of higher tension than seen in many years.

The US and Turkey are at odds over Syria policy and Erdoğan’s response to his coup

In the past year or so, the number of US grievances against Erdoğan has grown quickly. He has responded to an unsuccessful coup attempt against him in 2016 with extraordinary measures, jailing more journalists than any other world leader, arresting tens of thousands of Turks on suspect charges, and successfully pushing for a referendum that eviscerated legislative and judicial checks on his rule.

Erdoğan’s rising authoritarianism has had a direct impact on the US. The Turkish government has arrested dozens of American citizens while they’ve visited Turkey based on charges that they are linked to the coup attempt against the government. And Turkey has not been shy about targeting US consulate employees — last year, Turkish authorities arrested one, and on Monday they issued a warrant for the arrest of yet another one.

Turkey also rankled Washington by signing a deal to purchase a Russian surface-to-air missile system in September — a move that suggested Turkey might reduce its commitment to the NATO military alliance and potentially move closer to Moscow.

Washington has also been upset over the fact that last summer a Turkish state news agency revealed the location of 10 US military bases and outposts in Syria. Analysts say that Turkey leaked the locations out of anger over the US’s Syria policy — another major source of tension between the two countries.

Early on during the Trump presidency, the US aligned itself with a Kurdish-dominated force to fight against ISIS in Syria. Those Syrian Kurds are affiliated with Turkey’s Kurdistan Workers’ Party, or PKK — a separatist organization that Turkey considers a terrorist organization and an existential threat to the future of the country. Washington’s embrace of them has caused Turkey a great deal of angst.

And there’s another big issue that weighs heavily on Erdoğan’s mind: his demand that Gülen — the alleged coup instigator — be sent back to Turkey. The US is not extraditing Gülen and sending him back to Turkey despite Ankara’s pleas, and it is a constant source of frustration for them. American officials say Turkey hasn't yet presented persuasive evidence of his involvement.

Taken together, the travel restrictions are the newest blow, but they are unlikely to be the last one.

“What makes it so ominous is that it doesn’t seem clear how either side backs down from this,” Danforth says. “For Erdoğan domestically, so much of his persona and political identity is being a strong leader who stands up to the West.”

Trump also hates the idea of looking weak, or like he's buckled to a foreign leader. And so there’s good reason to believe this crisis likely won't end anytime soon.