I'm not sure I see the difference, besides semantics.
The difference between "Confederate offensives into Union territory
sometimes failed" and "Confederate offensives into Union territory
virtually always failed" is more than just semantics. The Confederates had a few successful raids, but when they advanced armies into into Union territory, the Confederates failed. Not sometimes failed;
always failed. When the Confederates concentrated superior numbers, their invasions of Union territory
always failed. When the Confederates sent their best generals, such as Lee and Longstreet, invasions of Union territory
always failed. When the Confederates sent their best generals
and concentrated superior numbers, their invasions of Union territory
always failed.
There's a difference between thinking the Union had a greater population and deeper manpower pools (and got on with mobilizing them for a long war starting in mid-1862) and thinking the Confederacy regularly won battles outnumbered 2:1. The Lost Cause argument, say, would hold that the Union Peninsular Campaign was a failure because One Reb Can Whup Three Yanks, not because the Confederates concentrated superior numbers at the point of decision.
You said that Confederate offensives failed "
largely because of the greater force the Union could concentrate against them". If your theory was correct, then the Confederacy would have a better track record for those offensives where they were able to concentrate greater force against the Union. But even when the Confederates sent their best generals
and concentrated superior numbers, their invasions of Union territory
always failed.
My argument was that the Confederacy's main problem was essentially relating to men (especially in the west), and that this could be alleviated in several possible ATLs - whether the effective destruction of the AotP during the Peninsular Campaign forces the Union to bring forces eastwards (reducing Union structural advantages), or the need to defend against the British and protect Washington results in a major drawdown in the west in a Trent war, or secession in the border states brings the manpower of the two sides much closer to even.
There will be no Peninsula Campaign in a Trent War setting; it would require McClellan to be insane to even suggest it when the Union is also fighting the British. The Union would have nearly twice the population of British North America and the Confederacy combined. More states joining the Confederacy is possible, but it would require a much less skilled President than Lincoln and the Union would still have roughly twice the population of the Confederacy. In either case, the Union will still usually be able to concentrate force against invasions. And even when the Confederates sent their best generals
and concentrated superior numbers, their invasions of Union territory
always failed
No, because your claim as stated implies far more troops being raised than "actual history" saw, at least in 1862. I have to guess at the number you're implying are being raised, because it looks like you're suggesting the Union could respond to All Confederate Forces in 1862 (425,000 Present, Livermore) and a British intervention (60,000 Canadian militia, 60,000 British regulars, 30,000 Maritimes militia) by raising "nearly double" that number in 1862, which by my count is approaching a million men Present - a number the Union never managed to raise all at once, as they peaked at 680,000 after years of buildup accompanied by a wide-ranging draft and massive bounties. If they'd raised a million men Present in June 1862 of OTL, they'd have had negative two hundred thousand small arms left (if not an even worse deficit).
Livermore
strongly disagrees with your claims that Union forces "peaked at 680,000" or that the Union never managed a number "approaching a million men Present".
Livermore, in
Numbers and Losses in the Civil War in America came up with:
* January 1862 Union troops 575,917 Confederate troops 351,418
* March 1862 Union troops 637,126 Confederate troops 401,395
* January 1863 Union troops 918,121 Confederate troops 446,622
Even if you cram 150,000 troops into Canada that still leaves the Union forces outnumbering the combined Confederate, British, and Canadian forces. The Union numbers could go significantly higher if they're willing to suffer the level of privation that the Confederates suffered.
That depends on how you view an offensive to be successful. It was a war between two countries in which one was completely defeated and so the loser never kept hold of any territory - but that's an unhelpful definition.
The Gettysburg campaign was fought with four objectives, of which Lee accomplished two - bring the war to the North and seriously bloody the Army of the Potomac. Would it count as a success if he'd also burned Harrisburg?
Lee's goals were to forage in Union territory, defeat the army of the Potomac, distract enough troops to stop Grant from taking Vickburg, hurt Union morale, encourage the northern peace party, and possibly to gain foreign recognition or even seize control of parts of Maryland. Lee only succeeded in foraging in Union territory and failed on all other points.
Of course, if the Confederacy is independent they've clearly done better. But it does not make sense to say "they were defeated OTL, they'd have to do better than that ATL, and that's ASB".
That's significantly misrepresenting my point. I said "The Confederacy would have to produce at least one general who equals or exceeds Lee just to gain independence. Keeping all of the 11 states that seceded is even more unlikely. The Confederacy gaining any bits of the Union Border states is nigh-ASB." It certainly is possible for the Confederacy to do better and is necessary for Confederate independence, but that requires that the Confederacy produce at least one more general who equals or exceeds Lee in skill. For the Confederacy to keep all of the territory of the 11 seceded states is a higher bar. Only Lee and Bragg managed to regain territory lost to the Union, and even then it tended to be "two steps forward, three steps back". The Confederacy made several attempts to seize and gain control of Union territory - West Virginia, Kentucky, Missouri, New Mexico, and Colorado. These attempts all failed, and usually failed miserably, even when the Confederates sent their best generals
and were able to concentrate forces
and were up against Union second stringers like Rosecrans. For the Confederates to seize West Virginia, Kentucky, Missouri, or New Mexico would require the Confederacy to produce a general that makes Robert E Lee look like Joseph Johnston
and can get along with Jefferson Davis. For the Confederate to seize most of them, let alone California, would require the Confederacy producing at least one general who makes Robert E Lee look like Leonidas Polk
and can get along with Jefferson Davis. Any Confederate general suddenly performing at the level Khalid ibn al-Walid or Subutai is wildly unlikely - for them to also be able to get along with Jefferson Davis is nigh-ASB.