Einmal ist keinmal
Non fui, fui, non sum, non curo
It's optional. The start player doesn't have to use the Polish Army to fight back anywere.
From p. 11 in the rules, "Poland Fights Back":
The first player is regarded as having the most influence with the King and is therefore the first to have the option of using the Polish army. He may select a region containing enemy strength cubes and attack them with the Polish army.
That's part of the game: deciding when to do what is right for the good of Poland vs. doing what is good for you only (oftentimes when this means it is detrimental to Poland).
I imagine you will encounter this often in this game. Try not to get hung up about it.