However, when the country in the early sixties purposed to remove prayer and bible reading, and embraced completely the situational ethics of expressing love, the U.S. has not been successful in completing a military campaign.
In my opinion, God removed his hand of allowing military victory from that time. For not a single conflict from Korea to the present has been completely resolved as was prior to the 60's..
More likely the United States stuck its nose in places it shouldn't have. (And this is to ignore that the War of 1812 was really a draw; the U.S. got most of what it wanted, but that was not because of military prowess. The UK had bigger fish to fry and, more importantly, didn't need to impress American seamen because it had neutered the French Navy.)
And was World War I "successful in completing a military campaign"? Certainly not in hindsight, given that it was recommenced 20 years later. You could ask Blackjack Pershing about his thoughts on the prosecution of the war.
The Mexican campaign before World War I was a dismal failure. Its only redeeming quality was that it trained troops who would fight in the Great War.
Certainly the United States eventually conquered the Native Americans. Surely God was strengthening the hand of those smiting the heathens.
He also seems to have paused in allowing successful campaigns in Grenada, Panama and the first Iraq war. (Admittedly, the second one was a mess and has led to innumerable difficulties that we are still living with.)
You seem to be forgetting the two unsuccessful invasions of Canada. Surely the Canadians were more righteous than the Americans.
Was the Korean War a failure? Ask the South Koreans, who have prospered and finally come around to democracy. Yes, the Kims are still in power and a menace, but I think the millions of South Koreans who have enjoyed a fruitful, if tense, existence would rather live in the South than under the Kims.
God uses nations to fulfill his purposes. They need not be moral to do so.