Which non-Communist country shared
a border with the Soviet Union in
northern Europe?

The Soviet Union was bordered by Afghanistan, Czechoslovakia (today Czech Republic and Slovakia), China, Finland, Hungary, Iran, North Korea, Mongolia, Norway, Poland, Romania, and Turkey, resulting in about 20,000 km of borders, additionally the country shared maritime borders with the USA and Sweden.

lincoln declared that slavery should not be allowed to spread into the new states, and of course this angered the south at the time.

the answer is mass marketing.