President Kennedy? If you look at his record he did nothing to be on this list, he is the most over-hyped president in history. LBJ did all the work, but gave Kennedy the credit. But as to your question, they were able to see a problem and look beyond their own concerns and convince people...
Of course there can be a just war. WWII was a just war. There can be no argument over that. There are plenty of wars that have been fought for trivial reasons. But such a broad statement can not be applied.