I landed in the New World and built a Settlement. Soon another European power met me. We agreed to be at peace, share World Maps and have Open Borders. Then they built a Settlement adjacent to mine. So I bought some cannon, created a couple of dragoons and set out to claim their land. I cancelled Open Borders, but got messages that the Peace Treaty 'can't be cancelled at this time' and 'Peace Treaty lasts 10 turns'. After 20 turns, I still couldn't attack, nor would Diplomacy let me declare War. What is going on?