The nuking of Los Angeles was a combination of physical models and 3D rendered shots. This close integration between physical models and 3D as well as using procedural techniques for the detonation was quite novel at the time. It was enabled by a new feature called "Mr. Nitro" written by Mark Granger, co-founder of Electric Image. The requirements of that groundbreaking sequence very much drove the features of Mr. Nitro, which was later released as part of Electric Image. EI 3D went on to render many shots for Star Wars: Phantom Menace, Star Trek: The Next Generation, Titanic, Men In Black, etc.

More info and images of both the physical and Mac rendered 3D models of Los Angeles being nuked: https://nccasymposium.bournemouth.ac.uk/2005/mscca/DIGGINS_h...