When you say that, do you mean McClellan accepted a formal truce (that is, he actively agreed to it)? Or just that no attack took place?
There certainly is room to discuss the "truce" and how "official" it was. In the end, no matter what he said then or later, McClellan made no attempt to attack or do anything else offensive. He made no effort to reject the "truce" that was, in reality, playing out on the field within his knowledge. I am sure you know this, so why bother trying to set up a strawman about it?
I agree McClellan didn't actually launch an attack, but my understanding is that McClellan wanted to attack if he could put together an attack that actually had a chance; the problem is that he had basically four divisions that were capable of attack (Morell, Couch, and 6th Corps) and Morell was over with Burnside who was complaining about being weak.
So, what, your position is that McClellan (the commanding officer) was subject to the whims of his subordinates? If Burnside is skittish, McClellan must refrain from attacking? I cannot agree with that.
You already know that both sides had suffered terribly the day before. You know what McClellan has -- and you know Lee has less. Lee's own generals thought Lee was taking a massive risk by not retreating on the night of the 17th-18th because of the extreme weakness of Lee's force.
If you are trying to say that there was a risk of failure if McClellan had attacked, then you are absolutely correct. If you are saying he could not attack because there was a risk -- then you are condemning McClellan's ability as a battlefield commander.
In which case, what is the date which fits the following conditions:
- Grant launched attacks on day X with a large part of his army, i.e. 75% or more which actually attacked on that day.
- Grant then launched attacks on day X+1 with a significant part of his army.
Let me guess: you have created an artificial scenario here that you think "justifies" your position and you want me to agree to play by your rules to "prove" what you want to "prove". What makes you think these points make any sense or show something important about real life? Can you show me some respectable military source putting forth this as a "rule" to evaluate by?
Can you show any two-day battle where McClellan attacks on both days? Any at all?
How many days does McClellan attack in the Seven Days, for example? How many days does McClellan attack at ...?
How about the Wilderness? Looks like Grant/Meade turns into Lee and attacks on May 5. Looks like Grant/Meade attack on the morning of the 6th, Longstreet counterattacks, looks like Grant/Meade counterattack Longstreet the same day.
Spotsylvania? May 8-9-10-11-12 there are three major attacks; when there are not, the artillery fire and skirmishing is sustained and frequent -- totally unlike the silence of the guns under McClellan on September 18 at Antietam. After four days of rain, we have more Union attacks on May 17-18-19.
What in the world makes you think this 75%-or-more-attack on two-straight-days rule means anything?
Note that what I am saying is that IF McClellan had attacked on the 18th then it would have been more aggressive (following a day of heavy attacks) than Grant under the same circumstances. It would not, for example, have been more aggressive than Pope (for whom 2nd Bull Run counts as two consecutive days of heavy attacks).
Note that McClellan decided -- no matter what he said or wrote to the contrary -- that he would not attack on the 18th. He offers excuses. He does not make a decision to attack. He struggles -- as he always did -- to make decisions like that. It seems to be against his nature, and he sees too much of what the enemy might do. Even worse, he worries far too much about what the enemy might do -- and he does not think enough about how seizing the initiative might help him and harm the enemy.