"diplomatic affairs" is a correct and usable phrase in written English.
You can use it when referring to the management and conduct of relations between two or more nations or entities. For example, "The ambassador was tasked with managing diplomatic affairs between our two countries."
When Prytz left in 1927, Quisling stayed on as the Norwegian diplomat responsible for managing British diplomatic affairs.
In diplomatic affairs, he was experienced, sober and sometimes sensible.
Hanotaux's historical interests centred upon early modern institutional history and contemporary diplomatic affairs.
His successor, John Grey Gorton, proved more assertive, especially of a sharper national interest in economic and diplomatic affairs.
But there is a context: an increasing assertiveness in global economic and diplomatic affairs by Chinese leaders.
It "just doesn't squeak as loud as the southern border," says Paul Frazer, a consultant on diplomatic affairs.
But Mr. Narayanan, in a rare display of blunt talk at a black-tie diplomatic affair, said that the description was "alarmist".
Margaret Thatcher's courting of the Soviet leader Mikhail Gorbachev was a complex diplomatic affair involving accusations of violence, unannounced visits and a strained Marxist joke, according to a top secret file released on Friday.
Among other things they have devised a diplomatic affair with mixed generations and familiar names to placate the fashion-conscious who missed having more of them around last time.
In the diplomatic cables affair, it deliberately chose to release certain materials and exclude others from the public domain.
In diplomatic and military affairs he proved to be arrogant yet cowardly, ambitious yet impractical.
