Overall though, any repeater that the Confederates are able to produce, the Union will be able to field a better weapon in much larger numbers. The frontage involved in the ACW prevents trench warfare from being the norm. Perhaps around strategic sites such as Richmond or Vicksburg, but overall the war would remain mobile. Where trench warfare arose the North would still have the advantage...superior numbers, better and more repeaters, better lines of supply and superior and more numerous artillery.
That might be true on the "strategic level," but as soon as the armies collide someplace, they will dig in, and the battle will be fought very much like Spotsylvania or Cold Harbor was fought in OTL. If you look at what happened in the 1864 campaigns...where armies moved from battlefield to battlefield, dug in, and then slugged it out...that is the kind of thing that will naturally occur in a situation where both sides have breechloaders.
As for the North having an advantage, you can look at what happened in Virginia in the OTL 1864 campaigns, Union casualties were MUCH higher than Confederate casualties...Grant lost more men during the campaign than Lee had in his entire army at the beginning of the campaign. Given that to win, the North MUST attack, while the Confederacy does not have to do so, the North WILL suffer massive casualties.
Causalities would be higher for both sides, which would work in favor of the Union.
Disagree. Casualties will be much higher for the side which attacks. Since that will almost invariably be the Union, it will not work in their favor.
Of course I don't see an aggressive commander like Lee sitting still in trench warfare for long, so massed attacks would still occur.
Lee might make that mistake once. But he was smart enough to learn from his mistakes. If you look at how he fought the 1864 campaign, it was all about conserving his forces while trying to lure the enemy into a position where he could be severely damaged. I would imagine that is how he would operate in this type of scenario.
In which case the attackers would suffer great losses until "storm trooper" or more modern bounding over-watch tactics are developed. And so in just a short amount of time we could see a Civil War that truly lives up to the idea that it was the first "modern war."
Benjamin
Indeed, that could very well happen. Emory Upton was working along those lines toward the end of the war in OTL.