Which is it?

The issue is back in the news with President Obama's decision to visit Hiroshima. The question that is being debated is whether he should apologize for President Truman's decision to order U.S. troops to drop nuclear bombs on Hiroshima and Nagasaki during World War II.

If the targeting of civilians in wartime is okay, then clearly the decision to nuke the people of Hiroshima and Nagasaki was not a war crime.

But if that's the case, then why was U.S. Army Lt. William Calley prosecuted during the Vietnam War? He's the officer who intentionally killed several defenseless women and children in a Vietnamese village. The military prosecuted and convicted him of a war crime. Why? If it's not a war crime to intentionally kill women and children in wartime, then why was Calley prosecuted and convicted of war crimes?