prevent an update from occurring under certain conditions

Hi,
I'm not sure where to look for information about how to accomplish this:

TABLES
state:
id | state_name | active
----+------------+--------
1 | Washington | t
2 | Illinois | t

county:
id | county_name | active | state_id
----+-------------+--------+----------
1 | King | t | 1
2 | Jefferson | t | 1

I want to prevent an update to set a state's active to false if there
are any counties in that state that are active (county.active = true and
county.state_id = state.id)
Is this possible?

Thanks in advance for your help.