For example two of the usual advantages, the Union had greater industrial capacity and the South had a military tradition in terms of leadership and experience since it was mostly southerners who fought in the war with Mexico, would not have been so distinct in say 1820 when the nation was younger and the war of 1812 would have left experienced veterans in the north as well.
The Confederacy cannot be said to have possessed a real advantage in military leadership for almost the entire war. And it certainly did not have such an advantage in the beginning of the war. Lee did not take command in the East from the indifferent Johnston until the summer of 1862, and his record prior to taking that command - an embarrassing defeat in West Virginia at the hands of none other than William Rosecrans - did not inspire much confidence. Albert Sidney Johnston spent 1861 and 1862 destroying what little chance the traitor states had in the West before his death at Shiloh. The CS Navy's best commerce raiding captains didn't get their starts until later in the war. And at the highest levels of government, Lincoln and the War Department were far superior in terms of overall strategic direction, command, and supply - Davis' rotating roster of War Secretaries were ciphers, and Davis himself refused to establish an actual overall
strategy for the Confederacy, a decision that plagued the traitor states throughout the entire war.
This lack of leadership is reflected in the grievous losses the Confederacy took during the first year of the war. By June 1862, Federal armies had captured New Orleans (the largest city in the Confederacy) and were at the gates of Richmond. They had evicted the traitors from Kentucky and from half of Tennessee, and were ranging throughout northern Mississippi. Federal amphibious landings had also seized control of key points along the North Carolina coast, and demolished fortifications in Georgia and South Carolina. To put it lightly, at that point the Confederacy looked like it wouldn't last the year.
Then McClellan threw away the Army's best chance at winning the war quickly, and his successors made a hash of things, either from incompetence (like Pope) or overpromotion (like Hooker and Burnside); the Federal offensive on the Mississippi stalled as the bluecoats outran their supply lines and had to solve a tricky terrain problem; and a group of new, energetic commanders for the Confederacy in the West - men like Bragg and Breckenridge - reopened the Kentucky issue and turned the walkover war south of the Ohio into a legitimate fight.
Only in that Confederate
annus mirabilis from June 1862 to June 1863 can one even
begin to speak of a possible advantage in certain levels of military leadership. And such an advantage cannot be reasonably said to be due to prewar expertise, certainly not Mexican service. Men like Lee, Longstreet, Jackson, Joe Johnston, Beauregard, and Stuart had fought reasonably well in Mexico, sure. But the Confederacy also possessed a surfeit of mediocre to bad generals with Mexican experience as well - the likes of John Winder and Albert Sidney Johnston. Furthermore, men like Breckenridge, who performed reasonably competently in the first stages of the war for the Confederacy, did not see combat in Mexico.
On the Federal side, many Mexican veterans did not pan out either, for various reasons. John Wool was a supernumerary who mostly just got in the way. Irvin McDowell and George McClellan fought in Mexico and ended up making complete asses of themselves. Of course, others served in Mexico and did well, such as Grant, Thomas, Hancock, Couch, and Sumner, all of whom made positive marks early in the war. Other quality Federal generals in the early going, such as Burnside and Rosecrans, did not serve in the Army during the Mexican campaign.
Simply put, the traitor states did not possess an advantage in military leadership for most of the war. Claims that they did are part and parcel of the so-called Lost Cause mythmaking - "we had the better leaders but y'all
cheated by havin' more men and guns" - and are not worth bringing up.
