Nicki Minaj -- Bad Case of the Benz ... Smashes Up BF's Mercedes

Now we know why Nicki Minaj and her longtime boyfriend broke up ... one of them was on the verge of going to jail after she smashed the windows of his Mercedes with a baseball bat.

Here's what we know. During the summer, Nicki and Safaree Samuels got into an argument. She started screaming, he got pissed, and she EXPLODED in anger, grabbing a bat and chasing him out of the house. She wisely chose property over person, and pulverized the 2012 Benz.

And for bad measure, she threw his clothes in the garbage -- just like in the movies.

Law enforcement sources tell us ... cops were called to the house, but no arrests were made. Safaree was escorted out of the house and left.

As for why Nicki didn't get in trouble ... it appears she owned the Mercedes, but allowed him to use it, and you can't get arrested for destroying your own property.

They broke up a short time later. Wise move.