The Western military alliance’s chief also suggested it put US and European troops at greater risk.
Trump said he had told allies he would “encourage” Russia to attack any Nato member that failed to meet the alliance’s target of two percent of their GDP.
Members of Nato commit to defend any nation in the bloc that gets attacked.
The White House called Trump’s comments “appalling and unhinged”.
Addressing crowds during a rally in South Carolina, the former president said he had made his comments about Russia during a previous meeting of leaders of Nato countries.
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