In Europa Universalis II, making and breaking royal marriages is a method by which a player can very cheaply raise relations with another country or countries. "Cheap" here is in terms of treasury money; the method is expensive in stability and diplomats, cheap in money. In general, making and breaking RMs is a good idea when the following conditions hold: Often you will need to pay money to bribe the other country through the "gap" in relations, from -5 to +25, where royal marriages don't work. But other than that, you can get relations all the way up to +200 if you have the time.