When reading Psalm 137:9 "Blessed is the one who takes your babies and dashes them against rocks."

I've seen many explanations for this and I seem to understand however I want an opinion from this community. Now depending on the translation will depend on how it is said. So for instance sometimes as're is translated as happy or blessed. So are the enemies happy that they dashed babies or are they blessed?

From what I understand this has fulfillment and is talked about in Isaiah 13.

What is happening here? I seek answers only so I can understand God better.

Whatever the answer is, I trust in God, believe in God, and he is good and Holy however I just want to understand God better.

Blessed be to you in the name of our Father, Son, and the Holy Spirit.

When reading Psalm 137:9 "Blessed is the one who takes your babies and dashes them against rocks."

Sometimes they’re translated as happy or blessed. So are the enemies happy that they dashed babies or are they blessed?

From what I understand this has fulfillment and is talked about in Isaiah 13.

What is happening here?