The Lord of Entropy, Ygorl used an eldritch machine known as The Anima Canon in order to tear apart the walls between the planes of existence. In this process, the Moon That Never Sets, which rained magic down on Eberron, fell from the sky and crashed into the Dragon Claw Mountains. The Breaking is the day that life as the world knew it, ended.
The fall of the Moon That Never Sets meant there was no ambient magic in the air, and thus killed all Warforged in one, extinction level event. In addition, the binding enchantment that held The Great Destroyer Harudogan was broken. At the same time, the Anima Canon torn apart the walls separating the various planes of existence, breaking down the one thing keeping law and chaos, good and evil, from engaging in ceaseless battle. To make matters worse, the deities that once ruled over these planes were weakened. Knowing that the release of Harudogan would mean the end of them and their worshippers, a majority of the weakened deities joined together to restrain Harudogan, leaving Ygorl unchallenged. A group of heros banded together to stand against Ygorl and his Slaadi army.
Balx, Poppy, Abadier, and Koyvs fought against Ygorl and ultimately won, killing the Slaadi Prince, but not in time to stop the Anima Canon from opening the walls of the planes, making one "Prime" plane. The Deities won against Harudagon, but at a large cost. Most major deities parished in the process. Tiamat and Bahamut fought on the Prime plane for 10 days, destroying most of the country of Cyre in the process. On the 11th day, they simply vanished, leaving a devastated landscape in their wake. No bodies, no victor, no hint as to where they went. The people of Cyre fled to Khoravaire leaving the country abandoned. The area was soon taken by one of the many displaced plane residence.
Denizens of the various planes fought for years. Humanoids have come to call this the "War of the Planes". Throughout the War, it was discovered that the Breaking did not destroy the planes, but merely removed the walls, making the Material Plane larger, creating on massive "Prime Plane". After 25 years, the fighting stopped and the denizens retreated to their ends of the world.
After The Breaking, Virages, massive chaotic evil beasts who once infested the Abyss, where now free to roam the plane aimlessly in search of dragons to fight with. The Chromatic Dragons, now all possessing the shapeshift ability, where at first happy to resist the Virages and defend their territories. But as the dragons began to realize that the Virages where an equal match for most dragons, and collectively decided that survival was more important then pride, and using their new shapeshifting ability, blended into the rebuilding humanoid societies.
Humanoids, quickly realized that with the abundance of new monsters roaming the lands, and of course the Virages still wandering, humanoids built dense cities, some several layers thick, with high walls and various perimeter defenses. New Warforged sub-races where created in mass to help protect the cities from threats. Out of the need for protection from threats, exploring the new Prime plane, and other hirable needs, a company called Acquisitions Incorporated worked with the world governments and created a license that allowed Adventures to work legally as mercenaries, granting them limited power of authority as long as they are a member of an Acqu. Inc. licensed Guild or employee of a Acqu. Inc. franchise.
City protection methods range wildly based on the wealth and population of the city from physical defenses like large energy canons, large armies of mages, and force fields, one city even teleports if an immediate threat attacks. As for the smaller cities, most have had to become creative such as befriending local giants, or lucky enough to have a dragon as a resident, but the vast majority of the smaller towns have perception filters that simply cause the town to be unnoticed by the less intelligent beasts.