One of the most important features in the game is Carribean Politics which you can view on your personal status sheet. This shows which countries are at war with each other.
The best way to make use of this is to wage war (pirate attacks) against the country that has the most enemies. For example if you the Spanish are at war with the French and the Dutch, is politically easier for you to gain military rank if you fight the Spanish than if you fight the French and the Dutch. Fighting more ships and plundering more cities also provide you with more "acre" bonuses each time you gain rank.
Mind you this tactic depends on the situation of Carribean politics and economics. Using the example above, if the French and the Dutch have better and more cities/towns (wealthier), it might be better to wage war against them instead since wealthier (and more) cities, this means they have more ships sailing than a country that has weak and less cities/towns.
So if you want to wage war against a country, remember not to always plunder a city to the brink of poverty as this will mean less ships to attack. Try changing the political powers of certain Carribean regions by capturing cities and giving them to nations that do not have a city or town in that region. For example in Eastern seas (around Martinique), the Spanish do not have a town there. The closest is Trinidad (South) and San Juan (North) but none in the middle. Providing a city/town for the Spanish in that area may prove beneficial for future political wars.