According to what Jan Długosz reports on the battle of Grunwald in the second half of the 15th century, the Lithuanians left the battlefield in disarray after a brief fight. The Swedish historian Sven Ekdahl found in 1963 an undated letter to the great master of the Order of Teutonic Knights where one can read: ‘It may so happen that your enemies will allow one or two banners to retreat or flee: that will be premediated and done in the hope that your formation breaks down since people typically like to mount a chase just as was the case in the great battle.’ Do you think that the letter cited undermines the reliability of Jan Długosz’s reporting?

Yes because the letter containing the rebuke must have been written shortly after the battle while Jan Długosz’s chronicle dates back to several decades after the event.
Hard to say, as the letter to the great master is not dated and its author unknown.
No. Jan Długosz is a reliable witness and both his father and the champion of his career bishop of Krakow Zbigniew Oleśnicki took part in the said battle.

Voting results for previous surveys.