In July of 1580, the Russian Tsar, feeling his backyard wasn’t large enough, decided to conquer Siberia. At the time, the territory was largely populated by a number of loosely-connected tribes under the taxation of Küçüm Khan. To retaliate at the intrusion, the Khan decided to force Islamic rule upon his people, and raised armies of Tartars to beat back the invasion.
The problem, however, was a fellow named Yermak. Nominally an explorer, he was much of the Spanish school of discovery which required any freshly encountered people to be hit with something heavy or sharp. His journeys went well, for him, and his expeditionary force of Cossacks and slaves quickly subdued everyone they happened upon.
The Tsar was quite pleased, and sent more men to help put down anyone who wasn’t fond of the new map. Everyone but the Khan was sure that Yermak had sealed Siberia’s fate, and it was just a matter of time before the last bits of resistance were stamped out. To reward the “explorer”, the Russian leader also gifted him a fine set of chain mail armour, an item that would make Yermak practically invincible to the weapons of the remaining Tartars.
It may have been the ease of his success, and the knowledge of his relative invincibility, that lead Yermak to folly.
From the wikipedia:
Küçüm Khan retreated into the steppes and over the next few years regrouped his forces. He suddenly attacked Yermak on August 6, 1584 in the dead of night and killed most of his army.
Now, to be fair, the army referenced in this snippet was just a portion of the total force that Yermak had spread over the Siberian countryside, and if he’d managed to survive the confrontation, he would have likely been able to rebound.
Unfortunately, for him, he did not.
Again, from the wikipedia
[…] Yermak was wounded and tried to escape by swimming across the Wagay river […] but drowned under the weight of his own chainmail.