I don't think Corbyn is a pacifist, actually. It would be interesting to see if he thinks that the UK was wrong to fight in WWII. He is certainly anti-nuclear weapons.
But these issues are normally misrepresented by the right-wing media, and presumably those who are hostile to Corbyn will feel yeah, he's a crazy lefty, and others will try to find out what he is actually saying, which often sounds quite moderate to me.