It's the Romans, primarily. The Roman Republic and Empire used eagle imagery in several related contexts that stemmed mostly from the eagle--a very large, powerful bird--serving as a symbol of the god Jupiter (roughly equivalent to the Greek Zeus). Jupiter's eagle appears all over the place, and was adopted by the Romans as a potent military symbol as well, with gilded eagles topping the standards of each legion.

The eagle standards were considered so important that they were treated like ritual objects, and losing them in battle was the ultimate disgrace (for an example, check out Quintilius Varus and the Battle of the Teutoberg Forest). Later empires seeking to emulate symbolically the image of strength and durability projected by the Roman Empire often adopted some form of eagle imagery, because it was such a prominent and effective part of Roman visual propaganda.