One of the most important things that made 1919 a year of "savage peace" was the red scare that was occurring in the United States at that time.
At that time, the Bolshevik Revolution had very recently turned Russia into the Soviet Union. It was known that communists believed that their ideology would come to take over the entire world and it was not unreasonable to think that it would come to the US. In 1919, there was a general strike in Seattle. There was a police strike in Boston. There were anarchist bombings that targeted important people. All of this led to a fear of communist subversion. These actions and the government's reactions (such as the Palmer Raids) helped make this a year of "savage peace."