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A note the entries for this volume are somewhat misleading in the sense that they only list the third edition begun by Professor Oppenheim in late 1918 and completed by Richard Roxburgh in 1920.
I am reading the second impression of the seventh edition of Oppenheim's International Law, Volume II Disputes, War and Neutrality, edited by Sir. Hersch Lauterpacht that appeared in 1955. This was to my knowledge the last edition readily available because, although Judge Lauterpacht intended to author an eighth edition the project remained incomplete at the time of his death in 1960.
Oxford University Press had stated that they intended to complete an eighth edition to be released during the same period as the ninth edition of Volume I, Peace edited under the careful watch of Sir Robert Yewdall Jennings, Judge Lauterpacht's successor as the Whewell Professor of International Law at Cambridge and like Lauterpacht a former judge on the International Court of Justice however, his death in 2004 prompted the abandonment of the project a second time.