The reason is that the rival systems represent rival religions and, ultimately, rival gods. R.J. Rushdoony points this out in Institutes of Biblical Law.
[I]t must be recognized that in any culture the source of law is the god of that society. (p. 4)
This is true both logically and historically. You can see war as a clash of religion and a test of the superiority of a nation's god(s) in the account of the Phillistines' capture of the Ark of the Covenant.
And the Philistines fought, and Israel was smitten, and they fled every man into his tent: and there was a very great slaughter; for there fell of Israel thirty thousand footmen. And the ark of God was taken. . . . (1Sa 4:10-11)
And the Philistines took the ark of God, and brought it from Ebenezer unto Ashdod. When the Philistines took the ark of God, they brought it into the house of Dagon, and set it by Dagon. And when they of Ashdod arose early on the morrow, behold, Dagon was fallen upon his face to the earth before the ark of the LORD. And they took Dagon, and set him in his place again. And when they arose early on the morrow morning, behold, Dagon was fallen upon his face to the ground before the ark of the LORD; and the head of Dagon and both the palms of his hands were cut off upon the threshold; only the stump of Dagon was left to him. Therefore neither the priests of Dagon, nor any that come into Dagon's house, tread on the threshold of Dagon in Ashdod unto this day. But the hand of the LORD was heavy upon them of Ashdod, and he destroyed them, and smote them with emerods, even Ashdod and the coasts thereof. (1Sa 5:1-6)
Getting back to Rushdoony's comment, we must answer the following questions:
- Judging from the source of her laws, who is America's god?
- Judging from her motives for going to war, what is America's religion?