During his successful presidential election campaign, Donald Trump branded NATO obsolete and demanded European countries pay their fair share.
Although he has since watered-down his rhetoric against the alliance, there remains the threat the US will withdraw support if Europe does not act on a pledge to spend at least two percent of GDP on defence.
So how do the US and Europe compare?
Latest figures show the US spent 3.61 percent of its GDP on defence in 2016, down from 5.29 percent in 2009.
European countries, by contrast, spent 1.46 percent of their GDP on defence. While lower than US spending, Europe’s outlay in 2016 grew for the first time in seven years.
Only four European countries met the two percent of GDP target in 2016: Greece, the UK, Estonia and Poland.