Complex sentence: The work of the young liberals has been known since the 1990's when they were active participants in the anti-war movement and the movement for a sovereign state of Montenegro, who were led by the Liberal Alliance.
Split into simpler sentences: The work of the young liberals has been known since the 1990s when they were active participants in the anti-war movement. They were also active in the movement for a sovereign state of Montenegro led by the Liberal Alliance.