While Europe's military heavyweights have already said that meeting President Donald Trump's potential challenge to spend up to 5% of their economic output on security won't be easy, it would be an especially tall order for Spain.

The eurozone's fourth-largest economy, Spain ranked last in the 32-nation military alliance last year for the share of its GDP that it contributed to the military, estimated to be 1.28%. That's after NATO members pledged in 2014 to spend at least 2% of GDP on defense — a target that 23 countries were belatedly expected to meet last year amid concerns about the war in Ukraine.