Complex sentence: In the late 1980s, when the Soviet Union, under its leader Mikhail Gorbachev, was embarking on political and economic reform, it began demanding payment from North Korea for past and current aid--amounts North Korea could not repay.
Split into simpler sentences: In the late 1980s the Soviet Union was embarking on political and economic reform. It began demanding payment from North Korea for past and current aid--amounts North Korea could not repay.