(establish) friendly (cordial) relations (with, between); (form) friendship (e.g. with a nation)
両市は友好関係を築くため、姉妹都市協定を結んだ。
The two cities signed a sister-city agreement to build friendly relations.
学校は長年の友好関係を保っている。
The school maintains long-standing friendly relations.
スポーツ交流が両校の友好関係を深めた。
Sports exchanges deepened the friendly relations between the two schools.