It was in the televised BBC debates during the 2017 election. And worse, I seem to remember it wasn't about retaliation, it was about whether he would likely destroy the world with a preemptive nuclear strike if Britain was ambiguously 'threatened'. Corbyn basically said no and the next half dozen questions from the audience were just pink faced men frothing at the mouth demanding he plunge the earth into a post-nuclear hellscape to ensure they never had to see a halal kebab shop.