[BroJoeK]

BJK: "This area was assigned to the United States by the 1786 Treaty of Paris."

Sorry, yet another typo.
The Treaty of Paris ending the Revolutionary War was signed in 1783 -- about 30 years before those British-built forts in US Northwest Territory were all finally evacuated by the Brits.

[JCBreckenridge]

Yep, the problem is that the treaty gave the Brits time to evacuate the forts at their own discretion. That, plus they were supposed to negotiate with the native tribes and compensate them, rather than the US which didn’t want to have to deal with it. These deficiencies were addressed in 1816.