It is possible that your Dad has something in common with the father in Calvin and Hobbes, whose deadpan explanations of gravity et al left Calvin confused and the audience amused.

However, treating the matter seriously, there is a historiographical question to be answered first: are there "decisive" events, and critically important people, that turn the flow of history into new channels? (The notion was probably kicking around for as long as people have reflected on history but first shows up overtly in Carlyle's lecture collection, On Heroes, Hero-worship and the Heroic in History, published in book form in 1841.) Think of this as "if you want real understanding, kings are more important than the peasants."

On the other hand, is it more accurate to say that "great men" and "critical events" emerge from the complex weave of the pre-existing order, and have an apparently crucial role when in reality, they are just surfing on the Great Wave of historical trends? "Before [the Great Man] can remake history, his society must make him." Spencer, The Study of Sociology," 1896.

(To illustrate: William invades England, and succeeds in conquest. If he had decided to attack a French neighbor instead, does that mean no Great Britain, no Empire, no industrial revolution, no modern world, because William and the conquest are critical to later developments? Or is it more likely that Britain's status as an island foredestined it to be a naval power and, that its position, protected from Europe's conflicts by a huge moat, substantially deterred invasion and encouraged continuous development? And that these and other, similar factors (good soils, abundant coal, etc.) made uninterrupted development into a superpower probable, even if William had never been born?)

When I was a history major centuries ago, variations on the latter idea dominated the field, and i believe they do today. Academic history, and historians as social scientists, are variously invested in social, cultural, and/or political history as the lens they look through. Amassing objective data, from tax rolls, land ownership, shipping records, etc. etc., in order to identify and analyze trends and conditions, exposes the reality of a time and place, and is thus far more important than the biographical study of kings and conquerors.

In that light it's hard to say that any event in the history of the Roman Empire (or any other) somehow made everything jump the track, remaking the future into something far different than it would have been without the event. A battle may change borders and trade routes, but won't change the suite of agricultural products being generated, or the state of metals technology.

However beneficial the newer historiography is, alas, it gets in the way of telling a good story. If we want to do that, we need to talk about kings and conquerors, battles and bravery. And it is no small thing that this kind of history has so much to do with forging national identity.

That said, Actium is a ripping good story even in dry narrative: The Republic has been swept away, Octavian avenges Caeser's death, Antony and Cleopatra flee and die in Alexandria, the period of civil war ends, all against a backdrop of a great naval battle. Shakespeare got two whole plays out of the material, Julius Caesar and Anthony and Cleopatra. And your Dad would argue, I'm sure, that the sweeping political events in the story, ending in solidification of Imperial rule, make it possible for Rome to conquer and subsequently dominate most of the known world. And how can you explain anything that happened later without showing how these dramatic events "set the table" for all that comes after? Props to your Dad.

Which story was he telling? What questions were you asking? If you're a bit more specific, we'll be able to help you.