The Left is anti-American. To be more precise, it's anti-Western. A lot of people simply see the term "anti-American" as a pure insult devoid of any objective meaning, like calling someone a "dickhead."

It does have a meaning, and it's exactly what it says. The Left has sided against the United States and its values and fundamental ideas in almost every conflict, military or otherwise, that I can remember. The ambition of the Left has been to diminish and dismantle the United States, economically, militarily, and in other ways.

If you want the motivation, here's one of several: for the crime of succeeding brilliantly while their beloved Marxism murdered every country it touched, America, capitalism, and the West have to be punished.