I don’t understand why any leftist would be against war.
The hard left wants socialism. Building socialism involves empowering the state until it is totalitarian.
But war does just that. In fact, it is the preeminent means to tyranny. As Rothbard put it:
It is in war that the State really comes into its own: swelling in power, in number, in pride, in absolute dominion over the economy and the society. Society becomes a herd, seeking to kill its alleged enemies, rooting out and suppressing all dissent from the official war effort, happily betraying truth for the supposed public interest. Society becomes an armed camp, with the values and the morale — as Albert Jay Nock once phrased it — of an “army on the march.”
What’s not for a leftist to love?