THE DAWN AGE – 12,000 YEARS AGO
The First Men arrive in Westeros via a naturally-occurring land bridge that connects the continent with Essos. Bringing their own culture and religion, they also bring along something that Westeros had never seen before: technology. Outfitted with bronze swords and boiled-leather shields, and utilizing horses for travel and combat, the First Men start to claim the land for themselves, chopping down forests to make room for their new settlements.
This displaces the original Westerosi inhabitants, a race of diminutive, magical beings that come to be called The Children of the Forest by the First Men; even worse, their revered weirwood trees, which they have the ability to warg into (and peer out of, thanks to the carving of faces into them), are systematically felled, which is sacrilegious to them. War follows.
The children employ their magic (including to smash the land bridge, transforming it into the Stepstone Islands), but they’re no match for the humans’ superior strength and numbers. In a last-ditch effort, they create the White Walkers to essentially act as a series of supernatural super-soldiers. However, this backfires, as the ice zombies, at some unknown point, turn against their masters, too.
A Pact is eventually signed which calls for the two peoples to coexist peacefully. The children agree to remain within the woods, while the First Men vow to leave the forests – particularly the weirwoods – alone. Eventually, they even adopt the children’s religion as their own, worshipping the gods of forest and stream and stone (a practice which remains in the north to this day).
THE LONG NIGHT – 8,000 YEARS AGO
A winter that lasts for an entire generation suddenly falls on the entire world, bringing with it the first all-out invasion of the White Walkers. Between famine, exposure, and the war itself, tens of thousands (if not more) of people perish.
In Westeros, the First Men seek out the now-forgotten dwelling places of the children of the forest, looking to forge a military alliance to keep the ice zombies at bay. It works, and the Wall – along with the Night’s Watch, to man it – is created afterwards, to ensure that a surprise attack can never befall the continent again. The children provide the first brothers of the Watch with several hundred obsidian daggers, the only weapon known to injure the Walkers.
The Iron Bank of Braavos, in the Free City of Braavos has become the most powerful financial institution in the Known World, with clients across Essos and Westeros, including the government of the King of the Andals and the First Men who rules over the Seven Kingdoms. The bank often reminds its clients who fail to repay their loans: "The Iron Bank will have its due". Currently, it is the only body to be in a position to restore Westeros to its past glory.
Largely unaffected by the War of the Five Kings, the House Frey and the House Arryn have remained powerful enough to be in contesting for the Iron Throne.
House Greyjoy through its powerful alliances in the past and due to the recent downfall of the Lannisters, is now in a commanding position and is also aiming to be named the Protector of the Realm. Though suffering the most loss due to the recent wars, the House Stark is still forceful and still very much likely to become the Lord of the Seven Kingdoms. There are also many other houses present in the realm of Westeros. Though, because of unceasing wars, all of the houses are at the mercy of the Iron Bank to survive in the race for the Iron Throne.
Daenerys Targaryen, having been made to leave Westeros, has established her dynasty in Essos with the help of her dragons and her aides, and earned herself the title Breaker of Chains. She is now returning to Westeros to take vengeance and claim the Iron Throne.
Underscoring all of this political squabbling is the real threat to the Westerosi: the white walkers, who are, after all these tens of thousands of years, returning to finish their assimilation of the living.