The second Canada intervened militarily, it was over. The CSA and the AUS had no chance after fighting each other to exhaustion over nearly a year in Virginia and Maryland. So when Canadian material flooded towards Denver, the Canadian Army stormed down Michigan and New England as the RCN made covert shipments from Europe impossible, the result was a given
Honestly on day one. People like too note about the famous generals who joined each the CSA or AUS but the vast majority of the rank and file along with thier equipment stayed loyal especially in the Army air crops and Navy which were the deciding factor in the war with the blockade of the CSA and with the US having air superiority for almost every part of the war especially the vital Mississippi River front and the USA winning every naval battle except the second battle of gaveston bay and even then that was a pyrrhic victory for the AUS with thier navy never leaving port again due the the damage and losses they took in that battle and that's not going into how badly the three CSA and AUS managed thier war time economy or the political infighting during the war. Victory for the US was only a matter of how long and costly it would be.
Combining these two answers gives us a clear picture.
The grave strategic mistakes the AUS and the CSA made on day one pretty much guaranteed their defeat.
The AUS and/or the CSA could have survived if they had played their cards right but ideology prevented that. With everyone going for the full price, trying to become the heir to the USA, it was guaranteed that the Feds would win because of structural, diplomatic, military and economic reasons.
1) CSA and AUS immediately lounged at each others throat. They were convinced that the "Ancien Regime" was basically finished and that the fight between the Revolutionary forces would decide the war.
Now we know that CSA and AUS were bleeding each other white, while MacArthur was busy shaping the U.S. military into a force to be reckoned with.
2) The CSA military basically was a labour union with tanks while the AUS military quickly became a militia with a few good generals.
a)The CSA never had many men with military experience to beginn with, their experiments with elected officers and soldier councils only made the situation worse. Leadership and training on every level were terrible and the only thing that kept them in the war was their material superiority. This material superiority wasn't as crushing as it seems as I will show.
Sure, French and UoB advisors helped but when the CSA actually started to listen it was a classic case of too little too late.
The CSA was never able to leverage its superior industry or conduct a modern war of movement. Later studies showed that the CSA military was unable to conduct combined arms offensives, that in average it took 4:1 superiority in infantry and 5:1 superiority in artillery to break through AUS lines and that friendly fire occured 40% more frequently than in the AUS military.
b) At the start of the war the AUS had a well lead military. However most of the technical specialists and most of the mid-level officers and professional NCOs followed MacArthur. So the ability of the AUS to absorb losses was fairly low.
The small AUS military at the start of the war won crushing victories against CSA militias and the Feds, who were in a state of chaos at this time. These initial victories already carried the fruit of defeat in them.
The AUS officer corps was bound by old southern honor rules and sustained the highest casualties among all participants of the ACW. After a few months losses already dilluted the quality of leadership.
Even more damaging was the rapid growth of the AUS military, young officers were promoted to early, experienced formations were torn apart to provide some backbone to greenhorns and new recruits weren't trained properly.
One year into the war general staff officers had to used as field commanders, two years into the war there weren't many people left that had the knowledge to properly handle artillery or how to service a tank.
Patton still tried to conduct sweeping offensives, he never understood that his army simply wasn't up to the job after the grueling losses in Virginia and Maryland.
3) The Feds actually had a working diplomatic corps and a bureaucracy that knew how to organize a state.
Good trade deals with Canada and Germany allowed the Feds to focus their industry. German optics, radios and high quality steel allowed the Feds to effectively produce high quality tanks. Canadian trucks and trains allowed the Feds to retain their logistical edge.
Sure, at the end the Feds were drowning in debt but neither the CSA nor the AUS had the trust to get credits in the first place. The CSA was basically isolated and had to rely on the UoB and the CoF, which both were busy with their futile arms race against Germany. The AUS had to pay for everything they wanted up front and in hard currency.
tl,dr: As soon as Huey decided that he wanted the whole cake he had lost. Huey could have edged out a little kingdom in the South, he got greedy and lost.
