There are a lot of elements of truth in that, Didz, but I don't believe the inevitability was necessarily there. I don't see that the south could conquer the north, but it was possible for them to win a limited invasion well enough to force the Union to agree to a peace. For the most part southern armies were better led. The key was that the largest, most critical southern army was well led, while for much of the war the largest Union army was not. The same cannot be said for the war in the west where it was a mixed bag on both sides.

A problem with people looking at the American Civil War is the focus on the east. Things were very different in the west. In the west the Confederate armies had been largely defeated long before. Leadership and quality of troops was more closely matched early in the war. In fact, Conderate armies were often larger locally (look at Missouri/Arkansas) and Union armies were not supplied properly. Western armies were as likely to be mishandled by the rebels as by the federals. Grant's successes in the west led to him receiving total command of the Union effort. Grant was a fighter. He bumbled at times and was not wary enough when in close proximity of the enemy, but he understood that to win, one had to fight. His tenacity was the key to winning.

If you look at the war in the west you get a different perspective of the men and armies: Grant's capture of Ft. Donelson, the battles of Shiloh, Corinth, the siege of Vicksburg. In the west there were setbacks for the Union (Chickamauga, the Red River campaign, various short sallies by armies in Missippi, etc.) Leadership and troop quality were closer in the west. The death blow to the Confederacy was dealt from the west with the march through Atlanta.

Because of the improvements in weapons it was easier to fight a defensive fight than an offensive one. Despite the Union's greater manpower it did need overwhelming might to be sure of victory. The Union had little in the way of a cavalry arm at the beginning of the war and was very much outclassed by the quality rebel cav, but the Union developed good mounted infantry by the end. The shift from cavalry mentality to mounted infantry was key. Fighting from horseback made one a very big target. But mounted infantry had the mobility to sieze key positions and hold them as infantry while the regular infantry came up. As the Union mounted arm improved, the rebels found it impossible to prevent heavy incursions and raids.

Fighting for one's own soil certainly makes a difference. Plus the changes in equipment had made defense easier. But don't forget that the defender also has a considerable advantage in scouting/intelligence as well--less "fog of war." The attacker must also spend more resources protecting his lines of communication and supply, while the defender can utilize local militia and the like as a supplement. Antietam and Gettysburg were both invasions by the south that ended in defeat and near catastrophe, and I think they illustrate the problems posed by invasion (by either side) quite well.