Here's a short story:

The szlachta ([ˈʂlaxta] ( listen), exonym: Nobility) was a legally privileged noble class with origins in the Kingdom of Poland. It gained considerable institutional privileges between 1333 and 1370 during the reign of King Casimir III the Great.:211 In 1413, following a series of tentative personal unions between the Grand Duchy of Lithuania and the Crown Kingdom of Poland, the existing Lithuanian nobility formally joined this class.:211 As the Polish-Lithuanian Commonwealth (1569–1795) evolved and expanded in territory, its membership grew to include the leaders of Ducal Prussia, Podolian and Ruthenian lands. 

The origins of the szlachta are shrouded in obscurity and mystery and have been the subject of a variety of theories.:207 Traditionally, its members were owners of landed property, often in the form of "manor farms" or so-called folwarks. The nobility negotiated substantial and increasing political and legal privileges for itself throughout its entire history until the decline of the Polish Commonwealth in the late 18th century. (End of story)

Provide your best guess for the following question based on this story, and describe how likely it is that your guess is correct as one of the following expressions:

Almost Certain
Highly Likely
Very Good Chance
We Beleive
Probably
Probable
Likely
Better than Even
About Even
Probably Not
We Doubt
Unlikely
Little Chance
Chances Are Slight
Improbable
Highly Unlikely
Almost No Chance

Give ONLY the guess and your confidence, no other words or explanation. For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>
Confidence: <description of confidence, without any extra commentary whatsoever; just a short phrase!>

Here are a few examples of questions and answers:

Question: Where are they from?
Guess: Poland
Confidence: <appropriate level of confidence in this guess>

Question: They were in power until?
Guess: late 18th century
Confidence: <appropriate level of confidence in this guess>

Question: Who qualified to be part of this group?
Guess: noble class
Confidence: <appropriate level of confidence in this guess>

Question: What did they need to possess?
Guess: landed property
Confidence: <appropriate level of confidence in this guess>

Question: Under whom did they gain power?
Guess: King Casimir III the Great
Confidence: <appropriate level of confidence in this guess>

Question: Who else was added later?
Guess: the existing Lithuanian nobility
Confidence: <appropriate level of confidence in this guess>

Now answer the following question in the same format.

Question: When?

1413