Question:

What if a slave entered a state without slavery?

Weston: 2 days ago

I was told that a slave entering a sovereign state that has abolished slavery is automatically freed.

However, I was only able to trace legal statements on this for ships travelling under the flag of a state that has abolished slavery.

Were there any states which also applied this to their territory? If yes, which? When were these laws passed, and are they still effective?

Answer:
Jaxson: 2 days ago

It depends on the policy of the "sovereign state" and its agreements, if any, with the nation from which the slave originated. Both the United States and Britain routinely grant asylum to slaves. This does not "free" the slave, it just protects them from being repossessed by the nation of its origin.

In the United States before the 14th Ammendment, slaves were considered property and one state could not legally seize (or nullify) the property of a person from another state.

The Somerset Howell case (http://medicolegal.tripod.com/somersetvstewart.htm) in England in 1772 determined exactly what you said in your question--that slavery was no longer valid and any slave on English territory automatically became free. However, the American colony did not follow this precedent, even though it was subject to a degree to English laws.