What is the theme in Poland by James A. Michener?

Another dominant theme of the book is the preservation of ethnic heritage and culture. The Poles always remained Poles and loyal to Poland, not matter who ruled them. The same is true of other peoples who were conquered and forced to live under foreign rule. They never lost their cultural identity throughout all of the invasions and years of foreign rule. Even when Poland didn't exist, having been totally absorbed by Austria, Germany and Russia, the Poles maintained their ethnicity and culture. Over the centuries, the Poles have been ruled by the Tartars, Swedes, Germans, Russians and Austrians, yet they never accepted the cultural identify of any of the foreign countries. T