A couple of years ago, I went to a talk by Shirley Williams, during which she explained why she wasn't a pacifist, as her mother had been. Williams, a practising Catholic, talked about Thomas Aquinas' "just war" theory.
"First, just war must be waged by a properly instituted authority such as the state. (Proper Authority is first: represents the common good: which is peace for the sake of man's true end—God.)
Second, war must occur for a good and just purpose rather than for self-gain (for example, "in the nation's interest" is not just) or as an exercise of power (just cause: for the sake of restoring some good that has been denied. i.e. lost territory, lost goods, punishment for an evil perpetrated by a government, army, or even the civilian populace).
Third, peace must be a central motive even in the midst of violence.(right intention: an authority must fight for the just reasons it has expressly claimed for declaring war in the first place. Soldiers must also fight for this intention)."
en.wikipedia.org/wiki/Just_war_theory
It seems the Catholic Church is beating itself up about "just war theory",because it thinks that the theory has been used to often to justify war.
www.catholicherald.co.uk/news/2016/04/15/just-war-theory-should-be-abandoned-says-conference-hosted-by-vatican/
Williams thinks that WW2 was justifiable, but WW1 wasn't. Of course, it could be argued that if WW1 hadn't happened, WW2 wouldn't either, but that's the realm of counterfactual history.
It's quite surprising that Corbyn's view should be so close to that of Catholic theologians.
I'm not 100% sure what is meant by being an "internationalist". If it means working towards world peace (probably unrealistic) and taking action when necessary and all other options have been exhausted, I don't see a conflict.