Cross Table Trigger

I trying to write a trigger base on the value in one table that a field can not be null when value is insert in a different table

Table 1
CountryCode char(3)
HasStates bit

Table 2
CountryCode
StateCode

When the go to insert or update in table 2, if HasStates = 1 then StateCode can not be null. I know I done it before but mind is drawing a blank

Thanks