> what was the threat they saved eachother from? the teutonic prussian? (be patient, i'm product of public education system) yeah, poland and lithuania cooperated and defeated the teutonic order in the 1410 (and later on as well). but especially poland helped lithuania in saving it while it was being invaded by muscovy/russia. then later on poland-lithuania invaded russia multiple times in revenge and for opportunistic reasons.    
DEUS VULT