Gondor
"The beacons of Gondor are alight, calling for aid. War is kindled."
Gandalf the White
Gondor is the closest Kingdom to Mordor and so much of it's energy is spent trying to keep the evil forces at bay. It was founded by Elendil during the Second Age of Middle-earth and was once a glorious and powerful realm.
Now, however, things have changed. Constant war with Mordor has reduced it's strength and population and now the realm is left with just one main stronghold, having lost Minas Ithil to the Nazgūl (Minas Ithil is now Minas Morgul). Osgiliath still remains to them, but barely. The site bridges the river and so much of it has been taken over by the evil things out of Mordor. Now it is used as little more than a barricade against Mordor and Minas Tirith - the main city of Gondor.
The realm of Gondor has been without a King for many ages. Elendil's line has failed and there is only one now who could reclaim the throne. Until that one returns the empire is ruled by a Steward, known as Denethor, the father of Boromir and Faramir. It is to him that Gandalf and Pippin now ride.