“This was made perfectly clear to the American military and forces from the very beginning. Therefore, neither the bases are authorized, nor, of course, is the use of Spanish airspace authorized for any actions related to the war in Iran,” Robles told reporters, describing the conflict as “profoundly illegal and profoundly unjust.”

Sánchez has called on the U.S., Israel and Iran to end the war.

“You cannot respond to one illegality with another, because that’s how humanity’s great disasters begin," he said earlier this month.

U.S. Secretary of State Marco Rubio said that Spain's leaders are “bragging" about cutting off its airspace, even as Washington has pledged to defend the NATO member. He said that the trans-Atlantic military alliance is useful for the U.S., because it “allows us to station troops and aircraft and weapons in parts of the world that we wouldn’t normally have bases, and that includes in much of Europe.”

“But if NATO is just about us defending Europe if they’re attacked, but then denying us basing rights when we need them, that’s not a very good arrangement,” Rubio told Al Jazeera on Monday. “That’s a hard one to stay engaged in and say this is good for the United States. So all of that is going to have to be reexamined.”

After Sánchez's government denied the U.S. use of the Rota and Morón military bases in southern Spain, U.S. President Donald Trump threatened to cut trade with Madrid.

Washington made trade threats last year, too, when Sánchez said that his government wouldn't increase its defense spending in accordance with a deal agreed to by other NATO members following Trump's pressure.

At the time, Sánchez's government said that Spain could meet its military commitments by spending 2.1% of gross domestic product on defense, instead of the 5% the rest of the 32-nation military alliance agreed upon.

Sánchez also has been among the most vocal critics of Israel's actions during the war in Gaza, which has invited criticism from Israel's government on several occasions.

No comment from NATO

Spain's new decision against a NATO ally is rare, though not unprecedented. NATO didn't comment, referring questions to national authorities.

"NATO allies operate with a presumption of cooperation, but of course they retain sovereignty,'' said Daniel Baer, director of the Europe Program at the Carnegie Endowment for International Peace and a former U.S. ambassador to the Organization for Security and Cooperation in Europe.

In an incident that strained trans-Atlantic ties, France and Italy blocked the U.S. military from using their airspace for an operation targeting Libyan leader Moammar Gadhafi in 1986.

In 2003, NATO member Turkey refused to allow American troops to use its territory to invade Iraq, though it did allow overflights. France and Germany firmly opposed that war, but allowed U.S. and British fighter jets to fly over their airspace.

France’s then foreign minister, Dominique de Villepin, despite a famed U.N. speech against the Bush administration’s plans to invade, told the French parliament at the time that “there are practices between allies that exist that we must respect, including overflight rights.”

Europe between a rock and a hard place

Spain's decision reflects broader concerns among traditional U.S. partners since Trump returned to office.

“The relationship with the U.S. was already strained,” Baer said. “Allies can generally be counted on, but they can’t be taken for granted.”

Still, he's doubtful that other European countries would follow Spain's example.

"Most Europeans are focused on keeping some measure of U.S. cooperation in supporting Ukraine, so I think it’s less likely that others join, even as they voice concerns about a lack of clarity around U.S. strategic objectives in Iran,'' he said.