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 will speak on Thursday at the Republican National Convention.
In a preview of what he will tell convention-goers in his speech, he outlined a foreign policy strategy aimed at reducing US expenditure and involvement abroad.
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.
The divisive Republican candidate also said that, if elected, he would not pressure US allies over crackdowns on political opposition and civil liberties, arguing that the US had to "fix our own mess" before "lecturing" other nations.
He said: "Look at what is happening in our country. How are we going to lecture when people are shooting policemen in cold blood?"
Asked about the failed coup in Turkey on Friday, the Republican candidate praised Turkish president Recep Tayyip Erdogan, who has been criticised by some Western leaders over his increasingly authoritarian rule.
"I give great credit to him for being able to turn that around," Mr Trump said of the failed coup. "Some people say that it was staged, you know that," he said. "I don't think so."
US Secretary of State John Kerry has urged Mr Erdogan to follow the rule of law, amid a crackdown on opposition figures by the Turkish leader in the wake of the coup attempt. But Mr Trump chose not to make a similar statement.
"When the world sees how bad the United States is and we start talking about civil liberties, I don't think we are a very good messenger," he said.