He has attacked the organisation as obsolete and characterised its members as ungrateful allies who benefit from US largesse. He says America can no longer afford to protect countries in Europe - and in Asia - without adequate compensation, suggesting he would withdraw American forces unless they pay up.
In one sense, he is simply voicing longstanding US concerns about most Nato members not meeting their goal of spending at least 2% of GDP on defence, while US defence spending is the largest in the world. He also stressed in a presidential debate he was "all for Nato".
But his stance has sparked alarm, particularly in eastern Europe which sees the alliance as a bulwark against Russia.