The only conceivable (and highly hypothetical) way a nuclear detonation over a city might not violate IHL would require a very narrow set of improbable conditions, which are highly theoretical and borderline implausible in reality.
Empty city scenario:
The city is completely evacuated (e.g. warning issued and civilians withdrawn). The target is a purely military installation (e.g. underground command center, missile base). A low-yield nuclear weapon is used with tightly controlled fallout and blast effects.
Extremis Self-Defense:
The attacking state faces an existential threat (e.g. imminent nuclear attack from another state). A nuclear weapon is used as a last resort, targeting a high-value military asset in a city. The attacker argues that this was the only available means to defend itself.
Hypothetical “Clean” Nuclear Technology:
Some future nuclear weapon is designed with minimal blast, heat, and no residual radiation. It targets a completely isolated, fortified military position within a city. Civilians have been evacuated, and the use is precisely calibrated to avoid harm.
But this is where the fantasy ends. Cities are full of civilians. Nuclear weapons cannot distinguish civilian from combatant. Blast radius, thermal radiation, EMP, and fallout affect vast areas, even with “low-yield” weapons. Hospitals, schools, water systems, food supply chains would all be devastated. Environmental destruction and long-term radiation would cause unnecessary suffering.
The attack would likely violate the prohibition of indiscriminate attacks, the principle of proportionality, and environmental protections (under Additional Protocol I).