[Squirrelloid]

You measure potential gain against the cost of taking it. Then you consider how easy it would be to keep it (how many new borders do you open up, how easy is it to get more units there to protect your new territory, etc...), what the political fallout is going to be (additional friends or enemies gained), and how long you think you'll have to commit forces to the war.

The best war is one that is fast, not very costly, lets you rapidly acquire territory/sites/key sites, and only gains you friends. This is why you often see a bunch of nations gank a nation, because you form ties with the others and the cost per nation is low on average.