Nuked and neutroned territories
When a territories is hit by either a nuke or a neutron bomb it is effectively removed from the game.
A hit by a nuke will destroy all forces and companies in that territory. A hit by a neutron bomb will destroy all visible boomers and up to 6 more forces, but leave companies intact. Land destroyed like this can be re-captured. Sea territories will revert to normal in the next Phase 1 - Pay Salaries.
Nuked territories can be recovered by a superpower with strategic tech level 5 attacking them - this is treated as an attack in terms of resource usage, but no forces will be lost.
Neutroned territories can be recovered (with any troops/companies left in them) by the Superpower that bombed them in the redeploy of the same attack, or by attacking them subsequently - with appropriate resource costs. Other superpowers with strategic tech level 5 can also recover neutroned territories. No forces will be lost attacking a neutroned territory.
Last gasp redeploy
If a superpowers home territories are all occupied/nuked/neutroned and the superpower has strategic tech level 5, they can redeploy into a nuked or neutroned territory to keep them in the game - normal redeploy resource costs still need to be paid.