People do not understand the direct link between the brutal treatment of animals in slaughterhouses and our present wars. Srila Prabhupada, the founder of Hare Krishna movement explains in these two quotes how the law of karma connects the two.
"In this age of Kali the propensity for mercy is almost nil. Consequently there is always fighting and wars between men and nations. Men do not understand that because they unrestrictedly kill so many animals, they also must be slaughtered like animals in big wars. This is very much evident in the Western countries. In the West, slaughterhouses are maintained without restriction, and therefore every fifth or tenth year there is a big war in which countless people are slaughtered even more cruelly than the animals. Sometimes during war, soldiers keep their enemies in concentration camps and kill them in very cruel ways. These are reactions brought about by unrestricted animal-killing in the slaughterhouse and by hunters in the forest. Proud, demoniac persons do not know the laws of nature, or the laws of God. Consequently, they unrestrictedly kill poor animals, not caring for them at all. In the Krsna consciousness movement, animal-killing is completely prohibited. One is not accepted as a bona fide student in this movement unless he promises to follow the four regulative principles: no animal-killing, no intoxication, no illicit sex and no gambling. This Krsna consciousness movement is the only means by which the sinful activities of men in this Kali-yuga can be counteracted."
SB 4.26.6 purport
"You are maintaining so many slaughterhouses, and when it will be mature, there will be war, the wholesale murder. Finished. One atom bomb -- finished. You'll have to suffer. Don't think that "Innocent animals, they cannot protest. Let us kill and eat." No. You'll be also punished. Wait for accumulation of your sinful activities, and there will be war, and the America will drop the atom bomb, and Russia will be finished. Both will be finished. Go on now enjoying. It takes time. Just like even if you infect some disease, it takes time. Not that immediately you infect, and immediately the disease is there. No. It takes a week's time or so."
Srimad-Bhagavatam 6.1.32 -- Honolulu, May 31, 1976