Complex sentence: The Situationist International (SI) was an internationalist group of revolutionaries based mainly in Europe with very restricted membership founded in 1957, and which reached its peak of influence in the general strike of May 1968 in France.
Split into simpler sentences: The Situationist International (SI) was an internationalist group of revolutionaries based mainly in Europe with very restricted membership. Founded in 1957, it reached its peak of influence in the May 1968 protests in France.