In 1976, Charles and Ray Eames released a short film tracing the outlines of the Roman Empire over centuries. The Eames film runs roughly five minutes. It was released to celebrate the bicentennial of Edward Gibbon's first volume in The Decline and Fall of the Roman Empire, the entirety of which runs just shy of 5,000 pages. If you have to pick just one, I recommend the five-minute version.
My favorite part: around 1:55, the Goths and the Huns get in on the action. The way the film runs, the map animation is narrated first, then the animation runs again without narration. So if you miss something on the map the first time, just wait -- you'll see it again.
For a more modern take on the same topic, here's John Green's Crash Course on the fall of the Roman Empire (in about thirteen minutes):