Spain's Prime Minister Pedro Sanchez defiantly posted "No to the war" on Wednesday, deepening a rift with the US after Madrid refused to allow the use of its bases to attack Iran and Washington threatened to cut off all trade.
Spain's Socialist Prime Minister Pedro Sanchez had already angered US President Donald Trump with a series of other policies.
Sanchez has refused to join Nato allies in a pledge to boost defence spending to five percent of GDP as demanded by Trump, and has fiercely criticised Israel's war in Gaza.
Trump lashed out at Sanchez's government on Tuesday, calling Spain a "terrible" ally and threatening to sever all trade with Spain.
Sanchez, one of a dwindling number of leftist leaders in Europe, defended his position on Wednesday, saying his government's position "can be summed up in four words: no to the war."
"We will not be complicit in something that is harmful to the world and contrary to our values and interests, simply out of fear of retaliation," he added in a televised address.
Spain is a member of the European Union, which allows goods to move freely between the 27 countries in the bloc. This would complicate any bid to impose trade restrictions on a single member state.
"Trump's words don't always become policy. We will have to see if he follows through, and how," said Angel Saz Carranza, director of the Esade Center for Global Economy and Geopolitics, a Spanish think tank.
European Council chief Antonio Costa wrote on X that he had called Sanchez to "express the EU's full solidarity with Spain."
"The EU will always ensure that the interests of its member states are fully protected," Costa said.
French President Emmanuel Macron also called to "express France's European solidarity in response to the recent threats of economic coercion targeting Spain," his office said.
US forces use the Rota naval base and Moron air base in southern Spain under an agreement signed in 1953 under the dictatorship of General Francisco Franco.
During the 2003 invasion of Iraq, Spain, then led by conservative prime minister Jose Maria Aznar, staunchly backed the United States by sending troops.
Spain's participation in the Iraq war sparked huge street demonstrations and many Spaniards blame it for the March 11, 2004 Madrid train bombings that killed nearly 200 people.
A branch of Al-Qaeda claimed responsibility for the attacks and called for the withdrawal of Spanish forces from Iraq.
Sanchez on Wednesday compared the Iran attacks to the Iraq war, which he said increased terrorism, increased energy prices and led to a less secure world. "We oppose this disaster," he said in reference to the Iran war.
The Spanish prime minister has emerged as a prominent figure for Europe's disillusioned progressives, who see him as one of the few remaining openly leftist voices in a continent increasingly dominated by right-wing politics.
His opposition to the use of the bases is seen by some analysts as an attempt to rally his supporters around an issue that unites the Spanish left.
Sanchez, in power since 2018, heads a minority coalition government that struggles to pass legislation.
The popularity of his Socialist party has taken a hit from a string of sexual harassment and graft scandals ahead of the next general election due in 2027.
Many on Spain's right consider Sanchez's opposition to Trump as motivated more by domestic politics than by a moral compass.
The head of the main opposition conservative Popular Party which tops opinion polls, Alberto Nunez Feijoo, accused Sanchez on X of using foreign policy for "partisan" purposes.
Left-leaning daily newspaper El Pais urged Sanchez in an editorial on Wednesday to "resist the temptation" to "exploit widespread hostility towards Trump in Spanish society to boost his popularity".
Agence France-Presse