As divorce became more commonplace and society understood that sometimes people just wanted to get divorced for reasons they did not want to make public, the grounds of irreconcileable differences arose. Simply put, the marriage was irretrievably broken, and both parties understood this. Moreover, while a divorce in Illinois requires that the parties be separated for 2 years for the grounds of irreconcileable differences to apply, parties can waive this requirement if they have been separated for more than six months.
However, if the parties do not agree that the marriage is irretrievably broken, then you have a situation where one party wants the divorce, and the other does not. If that is the case, then the party seeking the divorce can proceed on the grounds of mental cruelty. They must simply plead and prove, by their own testimony, that their husband or wife was mentally cruel or abusive. There is very wide latitude given to this definition, which is any action meant to cause any sort or degree of emotional distress, which can include any physical, emotional, or verbal abuse. The responding party is generally helpless to fight those grounds, and a divorce will surely be granted.