“Of all manifestations of power, restraint impresses men the most.”
Thucydides (460 BCE – 396 BCE) was an Athenian historian and general. His History of the Peloponnesian War recounts the 5th-century BC war between Sparta and Athens until the year 411 BC. Wikipedia

It isn’t actually Thucydides, but C19 author F.B. Jevons, discussing Ancient Greek prose style. The line is quoted, without attribution, in the introduction to an early C20 translation of Thucydides, and later readers then assume it *is* Thucydides.