An understanding of history is vital. How it moves, how it flows, how one thing supplants another. And American schools do a shitty job of teaching it, especially with respect to that more than one thing is happening at any given time. For example: the Sassanids are contemporary to the Byzantians. And considered co-equal by them. But I certainly (and I'm going to bet you, too) hadn't ever heard of them.

Some wonderful tools toward fixing that:

Interactive timeline of the history of Britain
(Spend some time between 100AD and 400AD and you'll be pleased you did.)

YouTube: The Civil War in Four Minutes
(Nicely keeps the mil-porn aspect of it in check with the running body-count in the lower right. Who knew 1.3 million Americans died in the war? Not in keeping with the theme, nessecarily, but quite well done.)

YouTube: Geographical History of Religion
(Only covers the big four, and only in broad strokes, but not bad at all.)

YouTube: Who Controlled the Middle East, and When
(Linked here before.)

Wikipedia: Sassanid, Seljuk, Mongol, and Assyrian Empires.