Complex sentence: The pro-Polish and anti-Prussian character of the painting caused William I, German Emperor to object to a proposal of rewarding Matejko; during this period Prussia engaged in attempts to suppress Polish culture on its territory and replace it with German culture.
Split into simpler sentences: The pro-Polish and anti-Prussian character of the painting made William I, German Emperor object to a proposal to reward Matejko. During this period Prussia was engaged in attempts to suppress Polish culture on its territory and replace it with German culture.