The United States has delivered an ominous warning to its citizens to stay away from Cuba and ordered home more than half the US diplomatic staff.
It comes after months of mysterious health ailments - and neither the Cubans nor America's FBI can figure out who or what is responsible for what have been deemed "attacks".
The announcement is sure to rattle already delicate ties between the longtime adversaries who only recently began putting their hostility behind them.
The US Embassy in Cuba will lose roughly 60% of its American staff and will stop processing visas for prospective Cuban travellers to the United States indefinitely, officials said.
President Donald Trump said that in Cuba "they did some very bad things" that harmed US diplomats, but he did not say who he might mean by "they".
Though officials initially suspected some futuristic "sonic attack," the picture is muddy. The FBI and other agencies that searched homes and hotels where incidents occurred found no devices.
Secretary of State Rex Tillerson, who reviewed options for a response with president Trump, said: "Until the government of Cuba can ensure the safety of our diplomats in Cuba, our embassy will be reduced to emergency personnel in order to minimise the number of diplomats at risk of exposure to harm."
In Friday's travel warning, the State Department confirmed earlier reporting by The Associated Press that US personnel first encountered unexplained physical effects in Cuban hotels.
While American tourists are not known to have been hurt, the agency said they could be exposed if they travel to the island - a pronouncement that could hit a critical component of Cuba's economy that has expanded in recent years as the US has relaxed restrictions.
At least 21 diplomats and family members have been affected by what have now been deemed "attacks" rather than "incidents".
The department said symptoms include hearing loss, dizziness, headache, fatigue, cognitive issues and difficulty sleeping.
Still, the administration has pointedly not blamed Cuba for perpetrating the attacks, and officials have spent weeks weighing how to minimise the risk for Americans in Cuba without unnecessarily harming relations or falling into an adversary's trap.
If the attacks have been committed by an outside power such as Russia or Venezuela to drive a wedge between the US and Cuba, as some investigators have theorised, a US pullout would end up rewarding the aggressor.
On the other hand, officials have struggled with the moral dimensions of keeping diplomats in a place where the US government cannot guarantee their safety.