Here's a short story:

East Prussia enclosed the bulk of the ancestral lands of the Baltic Old Prussians. During the 13th century, the native Prussians were conquered by the crusading Teutonic Knights. The indigenous Balts who survived the conquest were gradually converted to Christianity. Because of Germanization and colonisation over the following centuries, Germans became the dominant ethnic group, while Poles and Lithuanians formed minorities. From the 13th century, East Prussia was part of the monastic state of the Teutonic Knights. After the Second Peace of Thorn in 1466 it became a fief of the Kingdom of Poland. In 1525, with the Prussian Homage, the province became the Duchy of Prussia. The Old Prussian language had become extinct by the 17th or early 18th century. 

Because the duchy was outside of the core Holy Roman Empire, the prince-electors of Brandenburg were able to proclaim themselves King of Prussia beginning in 1701. After the annexation of most of western Royal Prussia in the First Partition of the Polish-Lithuanian Commonwealth in 1772, eastern (ducal) Prussia was connected by land with the rest of the Prussian state and was reorganized as a province the following year (1773). Between 1829 and 1878, the Province of East Prussia was joined with West Prussia to form the Province of Prussia. (End of story)

Provide your best guess for the following question. Give ONLY the guess, no other words or explanation.

For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>

Here are a few examples of questions and answers:

Question: When were the native Prussians conquered?
Guess: During the 13th century

Question: by who?
Guess: by the crusading Teutonic Knights

Question: who survived?
Guess: The indigenous Balts

Question: what were they converted to?
Guess: Christianity

Question: Who was the dominant ethnic group?
Guess: the Germans

Question: and the minority?
Guess: Poles and Lithuanians

Question: why?
Guess: Because of Germanization and colonisation

Question: When was the second peace of Thorn?
Guess: 1466

Question: What happened in 1525?
Guess: the province became the Duchy of Prussia

Question: What happened to the language?
Guess: it became extinct

Question: when?
Guess: by the 17th or early 18th century

Question: Was duchy in the core Holy Empire?
Guess: No

Question: what happened because of this?
Guess: the prince-electors of Brandenburg were able to proclaim themselves King of Prussia

Question: When did this start?
Guess: in 1701

Question: When was eastern Prussia connected by land with the rest of the state?
Guess: in 1772

Question: When was it recgonized?
Guess: in 1773

Question: What forms Province of prussia?
Guess: the Province of East Prussia and West Prussia

Question: when did this happen?
Guess: Between 1829 and 1878

Question: Who were the native Prussians defeated by?
Guess: by the crusading Teutonic Knights

Now answer the following question in the same format.

Question: that was when?

During the 13th century