I'm going to defend my girlfriend against a drunk neighbor
Why would you do that to your neighbor? What did he do?
He smashed his glass on the floor as I walked past him into the house
It sounds like a scary situation so it is good that you help defend your girlfriend. She will know that she will be able to depend on you.
Her ex-boyfriend has been coming by ever since she asked him to leave her alone.
Well, I hope that you are able to get your neighbor to calm down because it sounds like it must be terrifying for your girlfriend. It seems like you're doing a great job in defending her here.