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, they 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 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.