A metropolis is inherently vulnerable to interdiction, if you blockade all the supplies, drop a 500lb bomb on the water treatment plant/pipelines. Then wait for them to surrender or starve to death. If they have their own food, yummy yummy mil-rats, they will still suffer dehydration with no water.
People tend to forget that modern cities are not self sufficient, then rely on a steady input from food production areas (farms) and water processing, interrupt that and everybody dies. (and not quick and clean either)
A military installation might have it's own hardened water supply, but probably doesn't. They also might have sufficient food supplies of shelf stable foodstuff that everyone hates. But most military would need a really good reason to invest in such things, a good attacker won't give them a lot of advance notice. So what you are left with is IF there is a military base inside the metropolis (most are not) then it might have water available, otherwise you can kill them by denying water/food.