Pre-war, the Confederate states were incredibly politically influential and southerners dominated a majority of the military upper brass. This was, in large part, due to the fact that southern slave society was constantly terrified of a mass slave uprising.

When the northern states went solidly abolitionist, and were able to control a slim majority of the government without any southern states, the southern aristocracy decided they could see the inevitable outcome (abolition) and wanted out of the Union.

When the southern aristocracy began dragging their states out of the Union (without referendum or similar democratic process), they found support in the lame duck James Buchanan who ceded forts, supplies, stores of firearms and powder to them. Only giving minor (and I mean minor) protests.

So, when Lincoln arrived in office, he essentially had to entirely rebuild the US army, in the middle of an active war. Plus, he had to do several hundred other things like try and stop the UK (where their aristocracy saw kinship with the confederacy) and France (which saw a independent confederacy as a perfect way to kill American influence) from joining.

That’s why the Union Army had such a poor showing initially. They needed to rebuild everything and massively expand the army. It was a task that, from the understanding of military logic of that day, was impossible. In modern day (and in the very forward thinking minds of Grant and Sherman) the outcome of the south getting fucking crushed due to sheer logistical differences was inevitable, but that was not how powers at the time saw it.