Date: Signed 1768
This treaty was meant to resolve ongoing land disputes between the Indigenous peoples and ever-encroaching Colonists.
King George III had established a boundary line in 1763, but it had no basis in reality. Colonists were already living west of the line when it was drawn - not to mention the land wasn't really the King's to divvy up (I'm sure that just slipped his mind - Kings can be notoriously forgetful, the poor darlings, when it comes to the rights of others...).
I use the term 'negotiate' rather lightly - in reality, the Iroquois didn't have much choice in signing the agreement. It was give away some of their land and achieve peace (at least temporarily), or fight a long and drawn-out war. The question was only how much.
When the treaty was signed, it was - generous to Johnson to say the least - even handing over lands the British had left to the Cherokee earlier the same year. In fact, the Stanwix treaty was so controversial that the British-run Board of Trade ordered Johnson to renegotiate. He refused.
Johnson made a fortune speculating on land - I imagine this treaty was quite a windfall for him.