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 would be challenging.

President Donald Trump has called for NATO members to significantly increase their spending on defense, suggesting a target of 5% of gross domestic product to go toward military spending. What To Know: Trump reiterated the 5% of GDP spending goal on Monday following his inauguration,

For the boss of a military alliance dedicated to mutual security, Nato chief Mark Rutte seemed unperturbed by one member's recent threats to annex allied territory. On his first official visit to the European Parliament on Monday,