Barack Obama has accused Europe of being “complacent” about defence by failing to meet the Nato two per cent spending target.
It came as he delivered a passionate plea for the European Union to hold together, saying it is one of the “greatest political and economic achievements of modern times”.
In an address in Hanover, the US President said that Nato must spend more to address the crises in Syria and north Africa and to invest in new missile defence and cyber systems.
Nato is also increasing troop numbers and equipment in its eastern European neighbours who feel at risk of Russian destabilisation.
Mr Obama warned that states are failing to pay their “full share” of spending two per cent of GDP on the military. “I’ll be honest, sometimes Europe has been complacent about its own defence,” he said.
It follows remarks last month in which Mr Obama said European states are “free riders” by demanding the US act in North Africa without putting any “skin in the game”.
Donald Trump, the Republican presidential front-runner, has said Nato is “obsolete” and the US cannot “afford to be the policeman of the world.”
In Europe, the share of domestic spending on the military has fallen six years running, to an average of 1.4 per cent.
The US spends 3.6 per cent, while the UK spends just above two per cent. At the bottom of the table are Luxembourg (0.47 per cent), Hungary (0.85 per cent) and Spain (0.89 per cent).
Mr Obama urged Europeans to keep faith in the EU, as rising unemployment leaves many to ask “whether European integration can long endure, whether you might be better off separating off, redrawing some of the barriers and the laws between nations that existed in the 20th century.”
He said he understood that the EU requires “frustrating compromise” and “adds layers of government that can slow decision-making”.
But warning that a fragmentation of the EU would lead a resurgence of racist “us and them” politics, he said: “A united Europe, once the dream of a few, remains the hope of the many and a necessity for us all.”