"The United States has demonstrated not merely with words, but with its actions, that we stand firmly behind Article Five," Trump said, referring to the bloc's mutual defence commitment.
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Brushing aside a surfeit of transatlantic spats that have erupted since Trump took office in January, the US leader insisted that the alliance is in rude health.
"The transatlantic bond between the United States and Europe is as strong as ever, and maybe in many ways, even stronger," he told a crowd of 10,000 in the Polish capital.
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"A strong Europe is a blessing to the West and to the world," he said.
But he said that more defence spending was needed on the eastern side of the Atlantic, "for its own protection, and you know this. Everybody knows this. Everybody has to know this. Europe must do more."
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