Corbyn sure gets a lot of people's goat, at least judging by the sentiments expressed on this thread.
Yet he has been a very successful leader of the Labor party, which was in total disarray when he was elected. Just his keeping that entity together in the face of determined efforts to split it is a remarkable achievement by all accounts.
So he may similarly prove much more capable than perhaps people expect if ever he achieves power. How that translates to defense is murky, deliberately so, but with his experiences the man cannot be naive. I'd expect him to be anything but a pushover internationally.