Upon reading this I wondered why I always seem to get into complex examples. I can't imagine a more simply stated way to make a point.

Subject: Pacifist Lesson

What to do if you happen upon a peace rally by naive hemp-shirt-wearing college kids, to teach them why force is sometimes needed:

1) Approach student talking about "peace" and saying there should be, "no retaliation."

2) Engage in brief conversation, and out of the blue ask if military force is appropriate.

3) When he says "No," then ask, "Why not?"

4) Wait until he says something to the effect of, "Because that would
just cause more innocent deaths, which would be awful and we should not cause more violence." Or "America has brought it on itself, given its imperialist actiions and has angered the people of..."

5) When he's right in mid sentence, punch him square in the face as hard as you can.

6) When he gets back up to up to try and punch you back, point out that it would be a mistake and contrary to his values to strike you, because that would, "be awful and he should not cause more violence." In addition, tell him that his "actions angered you and that he brought it on himself."

7) Wait until he agrees that he has pledged not to commit additional
violence and that the reason you punched him was mostly his fault.

8) Just as he's finishing his apology, punch him in the face again, even harder this time.

Repeat steps 5 through 8 until they understand that sometimes it is
necessary to punch back.