In the year 1226, after the Old Prussians had destroyed the Polish Order of Dobrin which had been formed with the express purpose to subdue the Prussians. The Polish Duke Konrad appealed for help from the German Teutonic Order, and offered them in return for their assistance some of his lands, which they could use as a foothold for expansion into Prussia.
The conquest of Prussia took some 50 years of continuous bloodshed to complete. During which time native Prussians who remained un-baptized were subjugated, killed or exiled. Many battles between knights and Prussians were ferocious; in some lands the people were exterminated, leaving a barren land… no human life remained.
Similarly as Latvian and Estonian tribes, Prussians were not prepared to give up their religion, their lifestyle. In many cases they chose to die rather than surrender.
Those Prussians who tried to resist, went off to newly established pagan Kingdom of Lithuania, and continued their fight against these invaders, amongst the Lithuanians.
It wasn’t long before empty Prussian lands soon became inhabited by German settlers, and any remaining Old Prussians became peasants or were assimilated with the Germans, and vanished from the pages of history. Later German settlers started calling themselves Prussians, and so the Kingdom of Prussia was so formed.