Quite frankly the biggest enemies of poland were your neighbours, Islam was your enemies but that was centuries ago.
A pattern of abuse would make any country say enough is enough regardless of who the aggressor was (past, present and future).
The Earth has 36.5 billion acres of land. All of Poland consist of only about 77 million acres. So there is plenty of room for foreigns and their religious groups to reside elsewhere.
If you want to trade with Poland then great. Send us your money in exchange for our exports. You can come as a tourist for a short visit and take some nice photos and good memories of our exquisite food, rich history, and unique culture back home with you.
Why did Poland join?
Like other countries at the time Poland needed to restructure and grow its economy after decades of dystopian and dysfunctional communist occupation. It was also in Poland's security interests to free itself once and for all from Russia's orbit.
Even though Poland had to make very difficult domestic sacrifices at the time, the terms and conditions for EU membership seemed reasonable; i.e., to become part of a common market place which still respected national sovereignty.
There were no imploding economies within the EU until the PIIGS (Portugal, Italy, Ireland, Greece and Spain) began to squeal for bailouts. There was no manufactured "refugee" crisis with quotas being mandated for resettlement. All of that happened much later and had absolutely nothing to do with Poland's membership.
Brussels is to blame for all of it and now they want to socialize the pain and for Poland to be nothing more than a giant disposable sponge and absorb it.