Assassin-Templar War: The Knights Templar

"Even when your kind appears to triumph ... Still we rise again. And do you know why? It is because the Order is born of a realization. We require no creed. No indoctrination by desperate old men. All we need is that the world be as it is. And this is why the Templars will never be destroyed."

―Haytham Kenway, regarding the nature of the Templar Order, 1781.

According to the Assassin's Creed Wiki:

The Templar Order, also known as the Order of the Knights Templar, is a monastic military order-turned-corporate giant, which was formed during the prehistoric era. The Templars seek to create a perfect world, although their interpretation thereof - emphasizing purpose, order and control, over the freedom of individuals - directly contrasts the ideals of their sworn enemies, the Assassin Order.
Because of this difference in ideology, the Templars became involved in a covert war against the Assassins, spanning millennia and continuing into the modern era.

"Non nobis Domine, non nobis, sed Nomini Tuo da Gloriam"
"(Not to us God, not to us, but to Your Name Give Glory)"

―The motto of the monastic Templar Order.

In 1937, the Templars founded Abstergo Industries, a secret company aiming to control capitalists and workers and neutralize communism. The Templars, who had previously created the capitalistic economic system as a façade for their activities, hoped to control the people through owning corporations, as opposed to having people own the companies. The company would go on to control many corporations and organizations around the world, including the Ford Motor Company, NASA, the CIA, the US Federal Reserve, Kraft, Coca-Cola, and BP.