What is the main conflict in Mila 18 by Leon Uris?

Taking place during the Nazi aggression in Europe and continuing through the end of 1943, the bulk of the story occurs in Warsaw, Poland. The Germans defeat a weaker Polish army and occupy the country. Once entrenched, the Nazis begin the implementation of Berlin directives to "solve" the "Jewish problem" in Poland. Alex Brandel is a Zionist Jew who begins to write a journal, detailing Nazi activities, so that one day the world will know of the treatment and suffering of his people.