Complex sentence: Frederick's acceptance of the Bohemian crown in defiance of the emperor marked the beginning of the turmoil that would develop into the Thirty Years' War, which the English Parliament and public quickly grew to see as a polarised continental struggle between Catholics and Protestants.
Split into simpler sentences: Frederick's acceptance of the Bohemian crown in defiance of the emperor marked the beginning of the turmoil that would develop into the Thirty Years' War. The conflict, originally confined to Bohemia, spiralled into a wider European war, which the English Parliament and public quickly grew to see as a polarised continental struggle between Catholics and Protestants.