1. Following WWI, and reaching an apex during the Vietnam War, the Left has generally been hostile to anything having to do with war, often embracing pacifism.
When you start out your argument with a statement that isn't true, it's hard to take the rest of your OCD argument seriously.
A Democrat got us into WWI over Republican objections.
A Democrat got us into WWII over Republican objections. (Although Pearl Harbor sort of ended the debate.)
A Democrat got us into Korea, and it took a Republican to get us out.
A Democrat got us into Vietnam...
Now, true, AFTER Vietnam, the Democrats became more pacifistic. And unfortunately, that gave the GOP an opening with Nixon and Reagan to go back to doing what they did before- gutting the middle class and undoing the New Deal.
1. In the quote you provide,
"Left has generally been hostile to anything having to do with war, often embracing pacifism."
2. Many universities have departments of peace studies. The field of Peace Studies is alternatively known as Peace and Conflict Studies, Conflict Analysis and Resolution, and Peace and Justice Studies. It is concerned with the roots of conflict, the conditions for peace, and, ultimately, the daunting challenge of realizing peace on our little planet. To that end, if you major in Peace Studies, you'll read about and (hopefully) add to the large body of scholarship on the causes and prevention of war and how to create a more just and peaceful world.
Peace Studies
3. A central theme of Leftism is pacifism, largely because no welfare state can afford a strong military. Europeans came to rely on America to fight the worlds evils and even to defend their countries. This means that equality trumps morality.
4. Everything associated with the military is held in disrepute: nationalism, a strong military, honoring the military, referring to military dead as heroes. And even referring to anything as evil.
a. Since the end of WWII, the Left has opposed fighting almost any evil. Even when Saddam Hussein invaded Kuwait, the Left opposed military intervention. What could be more moral than opposing Saddams take-over of a nation, and considering the strategic importance of the area, and even the fact that the UN supported the use of the military
still, two-thirds of the House Democrats, and 46 of 56 Democrat Senators voted against the war.
b. Pacifism, the antithesis of nationalism, is a major attraction of both the United Nations and the World Court, both venerated by the Left. These vaunted institutions are opposed to all nationalism, except, of course, Palestinian.
5. The generalization of pacifism leads to the Lefts view of nationalism, and then to contempt for the idea of American exceptionalism, of an America which is prepared to use force to fight what it deems as evil, an affirmation of traditional Judeo-Christian values which include support for the death penalty.
And, of course that is the theme of the OP.