In my current game I wiped my three neighbours off my continent early and had a large landmass to settle. The other continent had Carthage, America and Rome who were mostly getting along. I wanted to rapidly expand my empire on my own continent without too much interference so implemented an isolationist policy to prevent them scouting out my land (my army was pretty thinly spread at points). With a large number of developing cities, compared to my rivals, Mercantilism worked very nicely until I could adopt State Property.