They invaded the Baltics as part of a crusade, and very quickly got approval to just annex for themselves the part of Poland belonging to their host Konrad I who they had been hired to serve. They just kept doing this and eventually built a state. Essentially they formed to exterminate "pagans" in Prussia and defend western Poland, and then conquered the Catholic land anyways and then invaded further and further all the way to their wars with the Novgorod Republic.

For a not exactly accurate but important snapshot of how that turned out watch the Eisenstein film Alexander Nevsky which dramatizes the famous Battle of the Ice and has one of the best scores in film history https://www.youtube.com/watch?v=HOgoGCP7Ujw It is such a blatant comparison to the Germans but it fucking works and goes HARD. They have soldiers wearing obvious stahlhelms. https://www.youtube.com/watch?v=Gq4PaJfod4w