Irretrievably Broken

Two complex words in lawyer speak that means a marriage is completely f*cked. But, how does one get to the point where a marriage is irretrievably broken? Is a marriage broken by infidelity? What if the infidelity spans the length of the marriage? Is a marriage broken by a lie? What if the lie is one of many and trust has ceased to exist between the partners? Is a marriage broken when fighting quickly escalates to talks of hate and divorce and disappointment? When the children cry quietly in their beds because the fighting scares them? When the years spent living apart more than triple than the number of years spent living together? I do not know at what point a marriage ceases to be a partnership and morphs into a battle fought with words and emotion but I do know once that threshold is crossed, there is no turning back.