Starting with an article by Vaclaw Lastowski in 1909, they proposed that the retreat had been a planned maneuver borrowed from the Golden Horde.[60] A feigned retreat had been used in the Battle of the Vorskla River (1399), when the Lithuanian army had been dealt a crushing defeat and Vytautas himself had barely escaped alive.[61] This theory gained wider acceptance after the discovery and publication, in 1963 by Swedish historian Sven Ekdahl, of a German letter.[62] Written a few years after the battle, it cautioned the new Grand Master to look out for feigned retreats of the kind that had been used in the Great Battle.[18] Stephen Turnbull asserts that the Lithuanian tactical retreat did not quite fit the formula of a feigned retreat; such a retreat was usually staged by one or two units (as opposed to almost an entire army) and was swiftly followed by a counterattack (whereas the Lithuanians had returned late in the battle).[63]