Russia's invasion of Ukraine and growing fears that President Donald Trump will withdraw US security guarantees that have protected Europe for decades have sent shockwaves through the continent.

Spain dedicated 1.28 percent of its annual economic output to defence in 2024, according to NATO figures, well short of the alliance's longstanding two-percent benchmark and less than all other members.

And in the new geopolitical scenario, with the European Commission looking to turbocharge defence spending to achieve greater security independence, the two-percent target agreed by NATO members in 2014 may prove insufficient.

Trump has repeatedly demanded other NATO countries bear a greater burden for Europe's defence and has floated the idea of raising the spending target to five percent of GDP.

For Felix Arteaga, senior researcher in security and defence at the Elcano Royal Institute, a consistent pattern in Spanish budgets explains why the country invests "little in defence".

"When the economy does well, the defence budget increases, but less than the other budgets. And when it does badly, it (the defence budget) is cut more than the others," he told AFP.

- 'Licking US boots' -

Socialist Prime Minister Pedro Sanchez reaffirmed his pledge to hit the two-percent mark during a visit to Finland on Wednesday and has mentioned the possibility of bringing forwards a 2029 target date.

But his coalition government faces an uphill battle as it lacks a majority in parliament and relies on an array of separatist and fringe parties to pass legislation.

Pro-independence Catalan and Basque formations are unwilling to bolster military spending and the Spanish army, while far-left groups are traditionally hostile to NATO and US policy.

Ione Belarra, spokeswoman for the hard-left Podemos party, said on Wednesday that they would not help the Socialists continue "licking the boots of the United States".

Even Yolanda Diaz -- a deputy prime minister from the Socialists' far-left coalition partner Sumar -- opposes boosting defence spending.

After Sanchez and Diaz met this week to discuss the thorny topic, a Socialist spokeswoman tried to assuage concerns, saying that "the welfare state will not be eroded" to stump up more money for the armed forces.

- Defence spending 'viewed differently' -

The 36-year dictatorship of General Francisco Franco long shaped perceptions of the military, but public opinion improved after the return of democracy, with the army participating in international peacekeeping missions and domestic emergencies.

But that has not translated into full support for more military spending.

In a November survey by national polling institute CIS, only 14.2 percent of participants backed raising the military budget "a lot more", with health and education emerging as the main priorities.

Spain stayed out of both World Wars and remained relatively isolated during the Cold War, which "has inspired our collective culture and led to defence spending being viewed differently", Arteaga wrote in an article.

Sanchez has steadfastly backed Ukraine, making several visits and welcoming 200,000 Ukrainian refugees to Spain. Last month he pledged one billion euros ($1.1 bn) in fresh military aid for 2025.

Spain earned a place at recent summits on the war in Paris and London involving a select group of European leaders, but maintaining that influence could depend on keeping up with allies on defence spending, Arteaga said.

If Spain "does not stand with the rest", the country will "lose influence in the decisions relating to defence and security", he added.