Guess.
If I have to guess, that the Polish-Ukrainian border is in the far west of Ukraine, I would guess the Poles are amassing troops on the border, as the Ukranians are likely getting as many troops to the east as possible and it is a little early for Russia to be amassing troops in land they have yet to occupy.
But perhaps it is the Germans. Or perhaps the French army hiding from the Germans. Or the Chinese. Or the Martians.
It’s your pronoun, why don’t you tell me?