The main reason that would make a country to end its participation in a war because it has become immoral is the presence of war crimes.
This refers to the inhumane actions that a person or soldier, usually fighting in a foreign country metes out on people, such as unprovoked aggression, extra-judicial killings, genocide, etc
Hence, we can see that the presence of war crimes is the main reason why a country would pull out of a war and claims it is immoral as this would have infringed on human rights.
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