BRUSSELS -- The success of a key NATO summit hung in the balance on Friday, after Spain announced that it cannot raise the billions of dollars needed to meet a new defense investment pledge demanded by U.S. President Donald Trump.

With armed conflicts involving states at a 70-year high, NATO is preparing for its most ambitious defence overhaul yet, but not everyone is on board. The next NATO summit kicks off in The Hague on Tuesday,

Spain has asked to opt out of NATO's plan to increase members' defence spending to 5% of their gross domestic product, a move that could derail a summit at which the military alliance plans to ask them to commit to the target.