Eagles have been a very popular symbol in many countries and cultures throughout history and have a huge amount of meaning attached to them.
When I think of Eagles being used as a symbol in a historical sense the first thing that springs to mind is the Roman Empire, they loved a good eagle and used them as a symbol of power and strength. The Romans also connected eagles with Jupiter (the God not the planet), while the Greeks associated the eagle with Zeus. The Germanic tribes connected eagles with Odin. So that’s some pretty respectable friends the eagle has there.
I was once walking round a falconry/zoo kind of place and walked round a corner just looking at the owls and there in front of me was an eagle. It was MASSIVE! It actually shocked me just seeing the sheer size and wingspan of this thing, then it started making weird, scary eagle noises and I was thinking, I would not mess with this dude! So I can see why they are such a powerful symbol of strength.
This is probably why so many nations and cultures such as The Spanish Empire, The Byzantine Empire, The Holy Roman Empire, The Russian Empire and The German Empire have all chosen to use the eagle crest.
Of course the bald eagle is widely associated with the United States of America in modern times due to the use of the eagle in the Great Seal and also due to the fact that the bald eagle is native to the continent.
So the main symbolism of the eagle is power and strength. As with all birds the eagle also represents freedom.