noun, noun or participle which takes 'suru'
restoration of diplomatic relations
Used in political and international contexts when countries resume formal diplomatic ties after a break.
両国は長い断交の後、ついに復交した。
After a long break in relations, the two countries finally restored diplomatic ties.
国交回復 is a more common and general term for restoration of diplomatic relations, while 復交 is a shorter, more formal or news-headline variant.
Compound of 復 (restore, return) and 交 (intercourse, relations). The exact historical derivation is uncertain, but the kanji combination directly suggests the meaning.