From 1866 the Machiavellian strategizing, faster small-arms, and more professional and modern military college in Prussia allowed that kingdom to progressively dominate German and other lands, with an (incomplete, lacking all of the German state of Austria, a charter member of the Kingdom of Germany and of the c. 19th century ‘German Confederation’…and including a good deal of Polish territory) official declaration of Empire coming in 1871. Still, that German state had the largest, boldest, best organized and most powerful land forces in Europe. Thus, the Imperial title was appropriate.
We disregard Hitler’s ‘3rd Reich’, and the ineffectual Imperial title the Habsburgs held after 1550 or so—but there was an equally powerful series of periodically unified German (technically, Austrasian/Frankish, their predecessors) empires from about 500. Chlodovech, Dagobert, and Charlemagne (and son Louis I) all briefly ruled huge empires largely through force of arms. However, there was less administrative unity and these empires were more personal unions of many kingships…and not precisely ‘German’ (or ‘Holy Roman’). Thus, we tend to start counting with the more sustained Ottonian dynasty’s power, 140 years after Charlemagne.