Spain's comeback

Solution 1:

So far, I haven't seen an invasion force originating from a Settlement and I'd be surprised to see that happen.

However, you can aggravate a single city by capturing several of their nation's ships going to or leaving from that port. If it takes too long, you can raise their anger by pelting their fortress with shots on the world map. To do so, hit "Space" to shoot at their fortress when you are close to the city.

Soon the port's fortress will take shots at you, meaning that the city has started treating you as an enemy, allowing you to attack it. Once you capture the city and your troops sufficiently outnumber the remaining defenders you may install a Spanish viceroy as the new Governor, giving Spain a new foothold in the colonies.

Note that, both, the ship blockade and the bombardment, reduce the wealth of the city.