Subject search terms to try in SOLO include:
International relations -- Decision making
Pacific settlement of international disputes
On aspects of the role & function of diplomats:
Diplomatic and consular service
Diplomatic and consular service -- Law and legislation
Diplomatic privileges and immunities
For diplomacy/international relations below nation state level try
Subnational governments -- Foreign relations
Municipal government -- International cooperation
Of course there are ebooks on this subject which holders of an Oxford SSO can access remotely - but SOLO may well direct you to the Law Library's near neighbour, the Bodleian Social Studies Library for the print version. Below is one example.
The Social Science Library (the Law Bod's near neighbour) is the principal Bodleian library for these subject areas.
Below is the Guide written by the Subject Librarian for these topics