I’ve visited a number of websites that offer explanations and examples of what is a war crime. A common rudimentary list of these is as follows:

murder

extermination

enslavement

deportation

torture

sexual or gender-based violence.

The United States could well be charged with commission of most of these crimes, not in war time, but in what the people in power agreed to call peace. 

The more obvious: 

The U.S. made repeated efforts of exterminate American Indians.

The U.S. encouraged the enslavement of blacks.

The U.S. caused the mass deportation of vulnerable populations, sc., the kidnapping and deportation of Africans as slaves and the deportation of Indians from property coveted by white America.

And now the Trump regime is plotting the mass deportation of a large part of the population of the United States. Of course, these likely deportees are not citizens of the United, but neither were the native inhabitants of America. This mass deportation will not be a response to pretended wartime necessity, but will be an indulgence of the fanatical racism and crippling selfishness of a mob of descendants of onetime immigrants to the United States.