Thucydides' History is extraordinarily dense and complex. His particular ancient Greek prose is also very challenging, grammatically, syntactically, and semantically. This has resulted in much scholarly disagreement on a cluster of issues of interpretation.
Strata of composition
It is commonly thought that Thucydides died while still working on the History, since it ends in mid-sentence and only goes up to 410 BC, leaving six years of war uncovered. Furthermore, there is a great deal of uncertainty whether he intended to revise the sections he had already written. Since there appear to be some contradictions between certain passages in the History, it has been proposed that the conflicting passages were written at different times and that Thucydides' opinion on the conflicting matter had changed. Those who argue that the History can be divided into various levels of composition are usually called "analysts" and those who argue that the passages must be made to reconcile with one another are called "unitarians". This conflict is called the "strata of composition" debate. The lack of progress in this debate over the course of the twentieth century has caused many Thucydidean scholars to declare the debate insoluble and to side-step the issue in their work.
The History is notoriously reticent about its sources. Thucydides almost never names his informants and alludes to competing versions of events only a handful of times. This is in marked contrast to Herodotus, who frequently mentions multiple versions of his stories and allows the reader to decide which is true. Instead, Thucydides strives to create the impression of a seamless and irrefutable narrative. Nevertheless, scholars have sought to detect the sources behind the various sections of the History. For example, the narrative after Thucydides' exile (4.108ff.) seems to focus on Peloponnesian events more than the first four books, leading to the conclusion that he had greater access to Peloponnesian sources at that time.
Frequently, Thucydides appears to assert knowledge of the thoughts of individuals at key moments in the narrative. Scholars have asserted that these moments are evidence that he interviewed these individuals after the fact. However, the evidence of the Sicilian Expedition argues against this, since Thucydides discusses the thoughts of the generals who died there and whom he would have had no chance to interview. Instead it seems likely that, as with the speeches, Thucydides is looser than previously thought in inferring the thoughts, feelings, and motives of principal characters in his History from their actions, as well as his own sense of what would be appropriate or likely in such a situation.
The historian J. B. Bury writes that the work of Thucydides . . . "marks the longest and most decisive step that has ever been taken by a single man towards making history what it is today.”
Historian H. D. Kitto feels that Thucydides wrote about the Peloponnesian War not because it was the most significant war in antiquity but because it caused the most suffering. Indeed, several passages of Thucydides' book are written "with an intensity of feeling hardly exceeded by Sappho herself."
In his Open Society and Its Enemies, Karl R. Popper writes that Thucydides was the "greatest historian, perhaps, who ever lived." Thucydides' work, however, Popper goes on to say, represents "an interpretation, a point of view; and in this we need not agree with him." In the war between Athenian democracy and the "arrested oligarchic tribalism of Sparta," we must never forget Thucydides' "involuntary bias," and that "his heart was not with Athens, his native city:"
"Although he apparently did not belong to the extreme wing of the Athenian oligarchic clubs who conspired throughout the war with the enemy, he was certainly a member of the oligarchic party, and a friend neither of the Athenian people, the demos, who had exiled him, nor of its imperialist policy."