Why did June and Bernard join the Communist Party?

In Part I, we learn that June Tremaine kept a framed picture by her bed of her and Bernard, taken only a day or two after their wedding in 1946, a week before their honeymoon would take them to Italy and France. The young couple is radiant, dressed in their business clothes, since because of the war their offices had not given them permission to leave immediately. They are not just leaving for their honeymoon, they've resigned their posts for good, which means they are free to officially join the communist party. "Beyond all their hopes for a sane, just world free of war and class oppression, they feel that belonging to the Party associates them with all that is youthful, lively, intelligent, and daring. They are heading off across the Channel to the chaos of Northern Europe, where they have been advised not to go." (pg. 5)