International law: origins & history: Earliest eras

Lex Rhodia Basilica. Liber 53
Rhodian sea-law

Maritime law -- Greece -- Rhodes
Maritime law -- Byzantine Empire
Maritime law (Roman law)

Maritime law (Islamic law)
Kitāb Akriyat al-Sufun wa-al-Nizāʿ bayna Ahlihā

MPEPIL: the Max Planck Encyclopedia of Public International Law  

has two entries  by Wolfgang Preiser which would be useful starting points 

• Basic Questions and Principles - an entry by Wolfgang Preiser
• Ancient Times to 1648, 

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.

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

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

Rome -- Foreign relations

Two major journals - became one

Archives d'histoire du droit oriental 1937-1951  Ancient Gen 300 A20 

Revue internationale des droits de l'antiquité 1948-1951 Ancient Gen 300 R20

merged & contd with double-barrelled title for 2 volumes 1952-1953
then continued from 1954 as just
Revue internationale des droits de l'antiquité 3e série Ancient Gen 300 R20
The Law Bod has print copies of t.1(1954) to t.46(1999).
Bodleian Art, Archaeology and Ancient World Library continues to subscribe, & has copies of all the above, and later volumes.

Various Bodleian Reading Rooms have titles published as part of Studia Amstelodamensia ad epigraphicam, ius antiquum et papyrologicam pertinentia. To find out which & where, search SOLO for this title as exact phrase but anywhere in the record (not as title)

Egypt -- Foreign relations
Egypt -- History

Middle East -- History
Middle East -- Foreign relations

Islamic countries -- Relations -- Europe
Europe -- Relations -- Islamic countries