Does it have to be detonated.
The threat by a terrorist group having one in any city can be used as blackmail.
A group makes a declaration in London that they have a bomb and want all Muslims imprisoned in the UK to be released.
Is the bomb real or imaginary given that a terror sponsoring organization has stated that they have made one (and/or) potentially tested it.
If Pakistan had a turnover and they stated that a weapon became unaccounted for then a group that operated out of Kashmir stated that they had the bomb in New Delhi, what would be the result.
Similar scenario.
Only it has not happened is because Pakistan has kept a close leash on its toys - possibly away from the ISI.
We do know that Iran has delivered increasing advanced weaponry to the proxies that it supports. Where does Iran draw the line, if it will?