Most common system in the world

I think it is democracy.

Explanation:

well this is going to be a long answer lol ok so the visigoths were a branch of the goths who frequently raided rome, uh the hunas attacked the guptas a lot and weakened them, uuhm of course the romans used math and science in there every day lives i mean they had a whole empire to run, and rome was split into east and west by a general named diocletian. hope this you! : )

to act quickly in times of danger to the united states

because you have to act quickly if you want to live/survive.