I play as correctly as i can. If a nation i'm not trading with attacks a nation i am trading with, i defend the nation i trade with. Not always by war on land, but by destroying their fleets and blockading their trade routes. If the war escalates and my trading partner is getting steamrolled i help out on land.

I never just invade somebody for expansion reasons, except in India and the new world where everything is fair game. Not other European factions there, i mean i invade the natives there purely for reasons of expansion.