Complex sentence: The forces calling for a Fifth International today are small, with the main proponent being the Europe-based League for the Fifth International, who campaign for its formation in the anti-capitalist movement and the international labour movement.
Split into simpler sentences: The forces calling for a Fifth International today are small. The main proponent is the Europe-based League for the Fifth International (L5I), who campaign for its formation in the anti-capitalist movement and the international labour movement.