Why did the civil war leave the United States government stronger than ever before?

Trick question. It didn't. The civil war pretty much wrecked the economy of the southern states. This lead to southerners being very poor farmers for generations. If they are talking about morally, then of course, the civil war made slaves free, and if they are talking about strictly politically, the civil war left the southern states in poverty, and allowed the north to rule the south with an iron grip.