Complex sentence: Prussia gained enormously in influence at the expense of the Holy Roman Empire, marking, it is argued, the beginning of the modern German state, an event at least as influential as the colonial empire Great Britain had gained.
Split into simpler sentences: Prussia gained enormously in influence at the expense of the Holy Roman Empire. This increase in Prussian influence, it is argued, marks the beginning of the modern German state, an event at least as influential as the colonial empire Great Britain had gained.