The time that what would eventually be Prussia was under the Polish and later the Commonwealth is unrelated. After all, Prussia took part in the partitions of the Polish-Lithuanians way before forming the German Empire. Both Austria and Prussia were major powers and both had a claim to Germany, so Prussia's victory over Austria in the Austro-Prussian war led to the Germanic states joining with Prussia and Prussia forming and heading the German Empire, the Second Reich. Poland had nothing to do with that, Prussia had everything to do with it.
The Second Reich and Prussia are quite similar in that both of them were extremely disciplined and extremely good at war. The Second Reich, in WWI, was easily the best military force in all of Europe (and that means the world). The only reason they didn't win was because the Central Powers were a whole lot of terrible allies and the allies had the Brits and French together, and even then they only won thanks to the Americans' fresh troops. Similarly, Prussia won the Seven Years' War (which lasted 9 years) despite being severely undermanned through epic victories and a good amount of luck. defeating both Austria (again in the Austro-Prussian War) and France, and surviving Russia and the Swedes. The seven years' war was basically Prussia with a bottomless fund of British cash against the rest of Europe. It's no wonder people consider Germany to be pure Prussian discipline.
Also, what does Prussia's abolition and the beginning of the HRE have to do with this?
