Could the South have won the U.S. Civil war

Definitely, yes. They had enormous resources and territory, and a substantial population. The problem is, their objectives were actually very confused. If all they’d really wanted was to be left alone, however, they definitely could have made it very much worth the North’s while to leave them alone. The best approach to this probably would have been not to formally secede, at all, but, simply become less and less cooperative with the federal government, and with the Northern States, in general. And, eventually, escalate to chronic guerrilla warfare throughout the South, so as to make it inconvenient and expensive for the North and the federal government to maintain a presence there. Eventually, the Union would simply have dissolved spontaneously, by mutual accord. It would have been an amicable divorce. However, what the South really wanted, was to take over the North, and, ultimately, the entire world. As a result of the immensely successful Mexican-American War, largely prosecuted by Southerners, the South had developed a bit of a God complex. They saw all Northerners as inferior to them, and, hence, suitable to be their slaves. Naturally, the Northern population was less than enthusiastic about this prospect! It had become clear to the North that it would be impossible live in peace with the South, because of the Southerners' delusions of grandeur. So, they accepted the need to go to war, to get the South back into line. Really, that's what the U.S. Civil war was all about. Not slavery, not preserving the Union, but, simply bringing the over-aggressive and self-aggrandizing Southerners back into line.