Answer
Yes, Kastler was a resolute pacifist. Even though his career flourished during the turmoil of the post-World War II era, Kastler remained vehemently opposed to war and violence. This pacifism was deeply rooted in his personal experiences during the war and his exposure to the horrors of modern warfare. Hence, Kastler actively engaged in peace activism and became a renowned advocate for peace and international understanding throughout his life.