I have commented briefly in the introduction (pp. xxvi–xxvii) on the very different circumstances in which what we think of as ‘texts’ were written and disseminated in the ancient world. We do not have an exact copy of the work that Thucydides wrote, either in its original form or with such revisions as he made himself as he proceeded with it over the course of twenty-five years or more. He did not live to complete the work or the revisions, and in this sense there never has been a single, authoritative ‘master text’, which we could in principle recover. Moreover, even in Thucydides’ day, the versions produced by successive scribes will have contained copying mistakes, which other scribes will have attempted to correct, so introducing further mistakes, and no two ancient copies will have been identical in every respect. Our own texts are derived from just a few medieval manuscripts that also differ from each other and are themselves derived from these earlier versions, many times recopied.

There is a huge scholarship on this textual tradition. See J. S. Rusten's edition of book II (1989), pp. 28–32 and his Thucydides, pp. 481–2 for a summary and a list of some of the main works.