Donald Trump has said that if he is elected president he may abandon a guarantee of protection to fellow Nato countries.
Speaking to the New York Times, Mr Trump said the US would only come to the aid of allies if they have "fulfilled their obligations to us".
Members of Nato have all signed a treaty that says they will come to the aid of any member that is attacked.
Mr Trump's comments hit at the fundamental basis of the Atlantic alliance; that an attack on one ally is an attack on all.
Under Article 5 of Nato's founding treaty, allies are bound to come to the aid of a member under attack.
The US has long been pressing its European allies to spend more on defence. That is slowly beginning to have an effect.
But never has there been a suggestion that the US would renege on its responsibilities.
His comments on Turkey suggest that the Republican contender also seems reluctant to insist upon Nato members maintaining strong democratic principles.
Mr Trump's positions will be seen by Washington's Nato partners as at best eccentric and at worst alarming.
At a time of growing tensions with Moscow, the idea that the US might become an unreliable ally is a nightmare for Nato's European members.
Asked about Russian aggression towards Nato countries in the Baltic region, Mr Trump suggested the US might abandon the longstanding protections offered by the US to such nations.
