Since the election of Donald Trump, commentators have rummaged through the past to make sense of the present and predictions about the future. They are preoccupied, in a word, by precedent. At the heart of any precedent is the belief that once acknowledged, it is actionable. A historical precedent offers not only a pattern but also a promise. We are assured by a rule — given what has preceded, here is how we must proceed — and reassured by a pledge: If we act rightly, we will be around to act again.

The Athenian writer Thucydides is often considered the father of scientific or objective history — the sort of history, in other words, pregnant with precedent. Modern scholars have described Thucydides’ account of the decades-long Peloponnesian War between Athens and Sparta toward the end of the fifth century B.C.E. — a struggle that ultimately spelled the decline and fall of both city-states — as a model of realism. Not only did Thucydides tell it like it was, but his telling also served as a blueprint for our own time.

The relevance of Thucydides seemed particularly great at the arrival of the Cold War. What better reflection of the conflict between the United States and the Soviet Union than that between Athens and Sparta? In one corner, a maritime and open society; in the other corner, a landlocked and closed society. In 1947, Secretary of State George Marshall claimed that Thucydides’ history offered the means to think “with full wisdom and deep convictions” about current affairs. A half a century later, the political theorist Graham Allison argued in an article that we still live in a world according to Thucydides, but with China now taking the role of Communist Russia.

Two decades later, in his 2017 book “Destined For War: Can America and China Escape Thucydides’s Trap?,” Allison asserts that the Greek historian provides a timeless rule — that war is more likely than not when a rising power challenges an established power. Allison thus seems to suggest the existence of apparently unchanging laws, first revealed by Thucydides, that — like Newton’s laws for physical matter — govern relations between great powers regardless of place and time.