"The Prince" by Machiavelli and "The Annals" and "The Histories" by Tacitus.
I would start there and then explore Clausewitz, Hobbes, Thucydides, perhaps a little bit of Polybius or Cicero.
But you really must start with Machiavelli and Tacitus. In fact, in the minds of many political realism/realpolitik start and stop with those two. Most everything else that has come after Machiavelli hasn't added anything new to the discourse, only restated with modern examples what he covered centuries ago.