The subject of Lucretius's six-book poem De Rerum Natura was not war, love, myth or history – it was atomic physics
Lucretius (full name Titus Lucretius Carus) lived in the first half of the century BC, probably from 99 to 55 BC. He overlapped chronologically with the political titan Cicero (who had read and admired Lucretius's work), and wrote during the tumultuous times that led, in the period after his death, to the collapse of the Roman republic and the establishment of the Roman emperors. His only work is De Rerum Natura, a six-book poem of roughly 7,500 lines, the beauty and power of which inspired allusion (the most literary form of flattery) and outright tribute in his more famous Roman poetic successors, including Virgil and Ovid. He wrote in a register of Latin that was self-consciously poetic, with occasional use of archaic vocabulary, and in the metre that since Homer had been the rhythm of epic heroes. But his subject was not, as we might expect, war, love, myth or history; it was atomic physics.
The title of his work reveals the ambition: De Rerum Natura is variously translated as "The nature of things", "On the nature of things" and "On the nature of the universe", a poem to explain the entire world around us. The choice of poetry as a medium for discussing and (as is Lucretius's stated aim) teaching physics might seem bizarre to us, but Lucretius did have some precedent in the pre-Socratic philosophers, who tried to explain the physics of the world, as several wrote in verse; most notably (for Lucretius), Empedocles had written a work, On Nature, setting out his physical theory (he believed everything was made from the four elements). The idea of a Latin poem about atomic physics jars us, however, not just because we don't naturally associate physics with verse, but because when someone mentions atoms, we tend to think of large hadron colliders rather than togas.
Several centuries before Lucretius was writing, however, some Greek thinkers had come to the conclusion that, if the world were actually to be able to exist as we perceive it, it would need to be made of some form of microscopic stuff that was in some way permanent. Atom literally means "indivisible"; Democritus and Leucippus first set out the idea of indivisible things (in response to ideas about the seeming paradoxes of divisibility most famously proposed by Zeno) in the 5th century BC. During the period that saw Alexander the Great rise to power, a Greek called Epicurus adopted and adapted that atomic theory for a very specific purpose: the promotion of human happiness.
"Epicurean" is a word that to modern ears implies (if anything) behaviour we don't tend to connect with modern physics: epicurean.com, for example, is "For food and wine lovers", and calling someone an Epicurean has, since at least the time of Milton, meant calling them an indulgent pleasure seeker to some degree. That meaning comes from the fact that Epicurus's philosophy is, at its heart, a hedonistic, or pleasure-seeking, creed; however, Epicurus believed that the greatest pleasure was simply to be free from mental distress, and that the surefire route to such a de-stressed soul was understanding atomic physics.
Lucretius tells us that Epicurus's belief in the human need for science was rooted in compassion: he looked around and saw a world full of people cringing in fear and dread of the wrath of the gods, as expressed via random phenomena such as lightning and earthquakes, which he aimed to teach them were in fact purely natural disasters (the legal shorthand "act of god" would have had his hackles rising). It was to appease that soul-crushing fear that Epicurus turned the atomic theory of Democritus and Leucippus into a means to provide a physics-based rationale of the world around us: if we understand the physics, we will see that we have nothing to fear from the gods. Epicureans were not atheists, but believed that the gods had no interest in humanity or our world. Lucretius' mission is to explain that physics in beautiful poetry, to make it more understandable and more palatable to his readership than its occasional philosophical obscurity might otherwise be.
Richard Feynman said that the sentence that contained the most scientific information in the fewest words was "all things are made of atoms". De Rerum Natura gives us that basic of physics, and a lot more besides: refutations of rival theories, explanations of mirrors and magnets, reasons not to fear death, some strong words about the folly of love, a mini-survey of human history and a range of causes for celestial and meteorological phenomena. Lucretius shows us the existence of invisible particles via the visible reality of the world around us, bombarding his reader with arguments and examples, to bring us what he believes is the truth of the universe and the key to contentment.