thumb|upright=1.1|Prussian Confederation offered to incorporate Prussia into the Kingdom of Poland, 1454, [[Polish Central Archives of Historical Records]]
The Prussian Confederation (, ) was an organization formed on 21 February 1440 at Marienwerder (present-day Kwidzyn) by a group of 53 nobles and clergy and 19 cities in Prussia, to oppose the arbitrariness of the Teutonic Knights. It was based on an earlier similar organization, the Lizard Union established in 1397 by the nobles of Chełmno Land.
In 1454, the leader of the Confederation, Johannes von Baysen (Jan Bażyński), formally asked King Casimir IV Jagiellon, to incorporate Prussia into the Kingdom of Poland. This marked the beginning of the Thirteen Years' War between the Order's State and Poland, with the cities co-financing the military costs of the latter.
Background
According to the 1411 First Peace of Thorn, which followed the Teutonic Knights' defeat in the Battle of Grunwald, the Teutonic Order had to pay high reparations to the Kingdom of Poland. The monastic state imposed high taxes on the cities to raise the funds as well as to re-arm for another war against Poland. In the 1420s, Grand Master Paul von Rusdorf brought stability to the Order and its relations, but fighting with Poland resumed in 1431, when the Knights' invasion into Poland during the Lithuanian Civil War sparked another Polish-Teutonic conflict.
Establishment
thumb|left|Foundation act of the Prussian Confederation from 1440
After about three decades of growing discontent, the burghers, nobility and landowners from the Teutonic state (see Prussian estates) organized themselves to oppose the rule of the order more effectively. The decision to establish the organization was made at a convention held in Elbing (Elbląg) on February 21, 1440. The convention was attended by representatives of the cities of Kulm (Chełmno), Elbing, Thorn (Toruń), Danzig (Gdańsk), Braunsberg (Braniewo), Königsberg (Królewiec), and Kneiphof (Knipawa) as well as nobles from various areas. On 14 March 1440, a group of 53 nobles and clergy and 19 cities, under the leadership of the Hanseatic cities of Danzig, Elbing, Thorn and Königsberg, founded the Prussian Confederation in Marienwerder (Kwidzyn). More towns joined later on (see Participating towns below). In Danzig, the new members signed a document
{| class="wikitable" style="font-size:90%"
|-
! Name
|-
| Thorn (Toruń) including "New Town"
|-
| Culm (Chełmno)
|-
| Elbing (Elbląg) including "New Town"
|-
| Danzig (Gdańsk)
|-
| Braunsberg (Braniewo)
|-
|Königsberg (Królewiec, now Kaliningrad), including Kneiphof (Knipawa) and "Old Town"
|-
| Graudenz (Grudziądz)
|-
| Strasburg (Brodnica)
|-
| Neumark (Nowe Miasto Lubawskie)
|-
| Löbau (Lubawa)
|-
| Rehden (Radzyń Chełmiński)
|-
| Wehlau (Welawa, now Znamensk)
|-
| Allenburg (Alembork, now Druzhba)
|-
| Zinten (Cynty, now Kornevo)
|-
| Heiligenbeil (Święta Siekierka, now Mamonovo)
|-
| Landsberg (Górowo Iławeckie)
|}
Towns which joined the Prussian Confederation in 1440:
{| class="wikitable" style="font-size:90%"
|-
! Name
! Date of accession
|-
| Starogard
| 31 March 1440
|-
| Morąg
|Rowspan=13|3 April 1440
|-
| Pasłęk
|-
| Miłakowo
|-
| Tolkmicko
|-
| Młynary
|-
| Tczew
|-
| Gniew
|-
| Old Town of Gdańsk
|-
|Nowe
|-
|Lębork
|-
| Łeba
|-
| Hel
|-
| Puck
|-
| Kwidzyn
| 17 April 1440
|-
| Chojnice
| 1 May 1440
|-
| Malbork
|Rowspan=15|5 May 1440
|-
| Bartoszyce
|-
| Sępopol
|-
| Rastembork (Kętrzyn)
|-
| Friedland (Frydląd, now Pravdinsk)
|-
| Orneta
|-
| Lidzbark Warmiński
|-
| Reszel
|-
| Dobre Miasto
|-
| Wartembork (Barczewo)
|-
| Jeziorany
|-
| Bisztynek
|-
| Olsztyn
|-
| Frombork
|-
| Melzak (Pieniężno)
|-
| Tuchola
| 6 May 1440
|-
| Kreuzburg (Krzyżbork, now Slavskoye)
|Rowspan=2|30 May 1440
|-
| Domnau (Domnowo, now Domnovo)
|-
| Sztum
| 21 June 1440
|-
| Gerdauen (Gierdawy, now Zheleznodorozhny)
| 18 October 1440
|}
Further towns joined in the following years, including Człuchów, Pasym, Nidzica, Działdowo, Dąbrówno, Olsztynek, Prabuty, Chełmża.
Further nobles joined following the outbreak of the Thirteen Years' War, and the towns of Dzierzgoń, Łuczany (now Giżycko) and Nowy Staw also sided with Poland in the war, and several more also recognized Polish rule and pledged allegiance to Poland, including Czarne, Debrzno, Golub, Kowalewo, Lidzbark, Łasin, Susz, Wąbrzeźno.