When is there an obligation to cease fighting?

 We have all heard of the "just war" teaching of the church.  Most people would agree that a war fought in self defense, such as is happening in Ukraine, fits the definition of just war. 

Here is a good summary of the catechism's discussion of war:

"2309 The strict conditions for legitimate defense by military force require rigorous consideration. The gravity of such a decision makes it subject to rigorous conditions of moral legitimacy. At one and the same time:

• the damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain;
• all other means of putting an end to it must have been shown to be impractical or ineffective;
• there must be serious prospects of success;
• the use of arms must not produce evils and disorders graver than the evil to be eliminated. The power of modern means of destruction weighs very heavily in evaluating this condition.

"These are the traditional elements enumerated in what is called the “just war” doctrine.

The evaluation of these conditions for moral legitimacy belongs to the prudential judgment of those who have responsibility for the common good."

Less clear cut that the first two conditions are the next two.  When is there an obligation to stand down, even when your cause is just?  When do people decide that the human cost in mayhem and misery is too great?  Can the US and other NATO nations help Ukraine broker a cease in hostilities which won't victimize them further?