It was a "Key issue" because it involved a Constitutional guaranteed property rights issue which were supported by Federal laws and United States Supreme Court Rulings and even an additional 5th Amendment added to that same Constitution protecting those same property rights.

Just because the Generetic term "slavery" is applied to this debate as a catch-all phrase does not make the mear ownership of a slave the sole issue of importance in that debate. To me Constitutional Rights, and abiding by the rule of law in a republic form of government, were just two of the far greater issues involved than was just mearly owning a slave. But the slavery debate is the debate that embodied both of those issues.

Just because some soldier wrote home that the Slavery issue was what caused the War does not mean that he was fighting to own a slave.