WASHINGTON—Donald Trump has provoked U.S. allies for years, questioning NATO’s relevance, demanding greater financial contributions and recently musing about the possibility of not protecting members who aren’t paying enough for defense.

       Now, with members of the alliance and other U.S. partners converging on Washington this week to celebrate the 75th anniversary of the North Atlantic Treaty Organization, heads of state are scrambling to find a common language with the Republican presidential candidate whose chances of winning in November have grown, according to the latest poll numbers.

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