MPEIL: the Max Planck Encyclopedias of Public International Law
has two entries by Wolfgang Preiser which would be useful starting points
The Law Bod has the print copy of this encyclopedia at Internat 500 E56c, but the online version has had a number of updates since the work was printed.
War and Peace in Ancient and Medieval History
by
Finding commentary
To find books with relevant material, you will probably have to move into subject searches such as
War (Philosophy)
Peace (Philosophy)
Alliances -- Philosophy
And when it comes to specific ancient jurisditions - their foreign relations, politics and government eg
Greece -- Foreign relations -- To 146 B.C
Greece -- Politics and government -- To 146 B.C
Athens (Greece) -- Politics and government
Athens (Greece) -- Foreign relations
War and society -- Greece -- Athens -- History
Roman Law and Maritime Commerce
by
Egypt -- Foreign relations
Egypt -- History
Middle East -- History
Middle East -- Foreign relations
Islamic countries -- Relations -- Europe
Europe -- Relations -- Islamic countries
Amarna Diplomacy : the beginnings of international relations
by