Complex sentence: Armenia gained independence from the Soviet Union in 1991 but faced enormous problems such as the 1988 Spitak Earthquake and the Nagorno-Karabakh War.
Split into simpler sentences: Armenia gained independence from the Soviet Union in 1991. It had faced enormous problems by the 1988 Spitak Earthquake and, soon after its nationhood, got involved in the Nagorno-Karabakh War.