Here is the end of Mill’s text, which in fact follows the quote above :
I am far from saying that the present struggle, on the part of the Northern Americans, is wholly of this exalted character; that it has arrived at the stage of being altogether a war for justice, a war of principle. But there was from the beginning, and now is, a large infusion of that element in it; and this is increasing, will increase, and if the war lasts, will in the end predominate. Should that time
come, not only will the greatest enormity which still exists among mankind as an institution, receive far earlier its coups de grâce than there has ever, until now, appeared any probability of; but in effecting this the Free States will have raised themselves to that elevated position in the scale of morality and dignity, which is derived from great sacrifices consciously made in a virtuous cause, and the sense of an inestimable benefit to all future ages, brought
about by their own voluntary efforts.
“The present struggle” was a civil War, Americans vs Americans. The precaution that Mill takes regarding the exalted character of this very war is omitted from the quote that started the thread. Mill only opposed to absolute pacifism : reducing all criticism against this war
as absolute pacifism seems a bit farfetched, to say the least.
There is a difference between discussing actions and debating about their principles. But there is a gulf (pun intended) between quoting and arguing.