No argument beats a strong Balkan temper.
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The rivalry begins in 1300s between Moscow and Lithuania. Poland had joined as an assisting force on the side of a bi-ethnic Balto-Slavic state "Lithuania".
You struggle to understand this because you see the area between Poland and Russia as a prize which we were supposed to share, when in reality it was a formidable force on its own. The Polish fulfilled their obligations by providing military assistance to Lithuania because they themselves needed a strong ally to assist them in their struggle with expansionist Germans who slowly but steadily pushed the ethnic Polish settlement area in a South-Eastern direction.
If you see these historical events as a "rivalry", then the descendants of our "rivals" are modern Lithuanians, Belarusians and Ukrainians. The role of Poles in their assistance against Russia is the same as the role of Belarusians in their assistance against Germans (on the Polish side).