The Hohenzollern successor state formed in 1525 when Grand Master Albert of the Teutonic Order converted to Lutheranism and secularized the Order's Prussian remnant into a hereditary duchy, held as a fief of the Polish Crown until full sovereignty was won in 1657 and united in personal union with Brandenburg from 1618.
Elevated to the Kingdom of Prussia on 18 January 1701, when Elector Frederick III of Brandenburg crowned himself 'King in Prussia' as Frederick I — the existing canonical 'Kingdom of Prussia' record (Q27306) picks up from this point (its own frames begin 1701).
The atlas holds 2 border frames for Duchy of Prussia — the shape you scrub through changes in 1525 · 1590.
No dedicated English-language monograph on just the 1525-1701 Ducal period was found on Open Library; Koch's single-volume political history is the standard English survey that opens with the Teutonic Order's secularization and Albert's 1525 duchy before tracing the state through to 1947 — used as a verified fallback distinct from the 'Iron Kingdom' title already attached to the existing Kingdom of Prussia record, to avoid citing identical reading across the two sequential polities.