To dow another country all one needs to do is to go into the dipomacy menu and select 'declare war.' You may have a non-agression pact with your neighbors which may stop you. I suppose you could also be out of money, but I doubt it.
Weird. I made sure i didn´t have non aggr pacts, and I have a couple of thousand in the bank,but still the 2declare war" option is greyed out....