Conversation so far:
What was a famous work of Paul Klee's that came out during the Mystical-abstract period?
Paul Klee created abstract works in that period such as Abstract and Farbige Kreise durch Farbbänder verbunden.
What other painters were famous during the Mystical-abstract period, besides Paul Klee?
Paul Klee produced watercolor paintings in the fashion of Paul Cézanne and Robert Delaunays' cubistic form concepts.

Question: Was World War I important in the Mystical-abstract period?
Self-contained rewrite: Was World War I important for Paul Klee in the Mystical-abstract period?