In a five-point statement, NATO endorsed the higher defense spending goal - a response not only to Trump but also to Europeans' fears of growing security threats.

The 32 allies' brief communique added: "We reaffirm our ironclad commitment to collective defense as enshrined in Article 5 of the Washington Treaty – that an attack on one is an attack on all."

NATO Secretary General Mark Rutte, who hosted the summit in his home city of The Hague, said NATO would emerge as a 'stronger, fairer and more lethal' alliance. The new spending target - to be achieved over the next 10 years - is a jump worth hundreds of billions of dollars a year from the current goal of 2% of GDP, although it will be measured differently.

Countries pledged to spend 3.5% of GDP on core defense - such as troops and weapons - and 1.5% on broader defense-related measures such as cyber security, protecting pipelines and adapting roads and bridges to handle heavy military vehicles.

The additional spending will be a tall order for European nations, many of which have strained finances.