I would really like to know which is the correct option: ‘The relationship between the two groups had suffered a complete breakdown /breakup.’
I would say ‘breakup’ because people can break up with one another whereas a car can break down.
Thank you for your time and support!
‘Breakdown’ in your sentence suggests that the relationship no longer worked and that’s the word that makes better sense here.
People within a relationship can break up and that means that they separate and go their different ways. That expression wouldn’t work in your sentence.