Part of a systematic strategy for destabilizing the border.

From 1492: The Year Our World Began by Felipe Fernandez-Armesto.  Russia has always been full of promise and an uncomfortable neighbor.  

Muscovy’s sudden and vertiginous rise took all Europe by surprise. The Saxon traveler and diplomatist Nikolaus Poppel, who arrived in Moscow in 1486, thought Ivan must be Casimir’s vassal. He was astonished to find that the Russian ruler had more power, more wealth, and possibly, by that date, more territory than the master of Poland and Lithuania. Fascinated, he contemplated the vast, open, exploitable lands that stretched to the Arctic, full of sable and copper and gold. But Ivan would not let him, or his successor as imperial ambassador in 1492, go there. In the Latin West, Russia assumed the mysterious renown of a fantasy land, an icy Eldorado full of strange wealth, with monster-haunted frontiers reaching toward the unknown. In the circumstances, Casimir might be forgiven for underestimating his eastern neighbor and neglecting the threat from Russia. He was always juggling conflicting responsibilities on other fronts, squeezing Prussia into submission, insinuating his brothers or sons into power in Hungary and Moldova, dueling with the Habsburgs for control of Bohemia.

Ivan could therefore go on provoking Casimir with impunity. As soon as Novgorod fell to the Muscovites, Ivan forbade Lithuanian enclaves within Novgorod’s territory from paying the taxes they owed to Casimir. In the 1480s, complaints lodged by Casimir’s envoys accumulated in Moscow: “thieves” from Muscovy were raiding across the border, burning and pillaging villages, sewing terror. Ivan professed ignorance and claimed innocence, but clearly the raids had his backing. They were part of a systematic strategy for destabilizing the border. Toward the end of the decade they escalated outrageously. In 1487, one of Ivan’s brothers occupied a slice of borderland on the Lithuanian side, and Ivan appointed a governor in districts traditionally part of Lithuania. A raid in 1488 carried off seven thousand of Casimir’s subjects. Many border towns reported repeated raids between 1485 and 1489.