I'm fairly certain it's ironic juxtaposition. We'll Meet Again was a massively popular wartime song, sweetly sentimental, asking people to keep their spirits up

I'm fairly certain it's ironic juxtaposition. We'll Meet Again was a massively popular wartime song, sweetly sentimental, asking people to keep their spirits up, giving them hope their loved ones will come back after the war and all will be OK. The montage of explosions is a visual bombardment with a diametrically opposed message. It's the perfect combination.