I like Thucydides, and I can recite the Melian Dialogue from heart. But I also think that as much of a classic the book is, it has become too totemic a work of literature for both classical realists and neorealists. So I have some alternate reading and viewing suggestions for realists that also illuminate aspects of strategy and international relations.
1. The Romance of Three Kingdoms
Classic Chinese epic novel about the struggle for power among a multiplicity of warlords. The level of violence, duplicity, and pure drama here dwarfs anything that Thucydides could ever imagine. And Cao Cao is the kind of character Gary Oldman would win an Oscar for if he were Chinese.
2. Dune series
Control over precious resources. Interplanetary geostrategy. Warring royal families. Religious prophecies and fanatical cults and organizations. Byzantine conspiracies of immense complexity. Dune simply has it all. You should be equally familiar with Houses Harkonnen and Atreides as you are with Pericles.
Xenophon's classic tale about a group of Greek soldiers who find themselves stranded when their Host Nation undergoes a drastic political shift is not just a tale of military exploits but also a story of strategy and diplomacy as their leader methodically leads them up back to Greece through Persian lands. An instructive read for the "expeditionary" era.
4. The Godfather, Part 2
Several realists have recommended the first movie, but I think the second is far more interesting. Why? It's a story in, part, of maturing power as Michael Corleone gradually grows into his role as top boss of the Corleone family. It covers the difficulties of a grand strategic shift, as Corleone attempts to move his operations West and become more legitimate. It's also mainly very interesting for its deep psychological insight into a leader's decisonmaking calculus and the burdens that strategy--especially in a world as anarchical as that of the mob--inflicts on a leader who must set policy.
5. Zulu Dawn
The prequel that no one ever watches to Zulu. Instead of the heroic defense of Rorke's Drift against the Zulu hordes, it shows a complacent British army being annihilated at Isandlwana. It also depicts, in sad detail, the political machinations that ensured that an ill-prepared force would be sent into Zululand without a clear strategy for victory. There is a reason why this film is obscure--it is a very, very depressing story but just as relevant as the similarly downbeat Battle of Algiers.
6. The Seven Samurai
Akira Kurosawa's classic is ultimately about the defense of a political community. Machiavelli advocated militia forces to preserve Republicanism in his home city-state and protect it from outsiders, and wrote very strongly in both The Prince and The Art of War on the kinds of political motivation it would take to develop such forces. The Samurai, who both mistrust and are mistrusted by the villagers (who have a history of antagonism with Samurai) have to not only create a popular force to defend against a superior foe but also motivate it.