Complex sentence: The Congress of Estonia was a rival parliament set up in the Estonian SSR during the process of Estonia's separation from the Soviet Union, to challenge the power and authority of the Supreme Soviet of the ESSR.
Split into simpler sentences: The Congress of Estonia was a rival parliament set up in the Estonian SSR during the process of Estonia's regaining of independence from the Soviet Union. It also challenged the power and authority of the Supreme Soviet of the ESSR.