That is correct. Remember to not do a straight line, but any square that is touching that harbor. Even one tile is enough to let the trade through.
There is a down side that I forgot to mention, and I'm glad I looked back into this thread. You have to cut off all contact with any other civ, as the trade route will look for any way to get from the captiol (or resource/trade) to the city. Harbors, roads, airports. So if the city is connected by road to another city regardless of civ nationality that is connected to a harbor or airport, then the trade will still go through so long as the civs are at peace. So if you're trying to blockade France, and France is neighbors with England, and they are not at war, the French can and will use the English Harbors to get the trades through. So if they are connected by roads not in a territory, chop them, or blockade the English Ports too.
Better, if you're at war with them, then chop the roards next to that city as well as blockading them. This will serve the same purpose. And if you chop off the roads around the captiol, say with a nuke or some such, then the resources that AI is trading will go bye-bye. Unless the captial is a port city, and has an active harbor.
Get's complicated pretty quickly, doesn't it?
There is a down side that I forgot to mention, and I'm glad I looked back into this thread. You have to cut off all contact with any other civ, as the trade route will look for any way to get from the captiol (or resource/trade) to the city. Harbors, roads, airports. So if the city is connected by road to another city regardless of civ nationality that is connected to a harbor or airport, then the trade will still go through so long as the civs are at peace. So if you're trying to blockade France, and France is neighbors with England, and they are not at war, the French can and will use the English Harbors to get the trades through. So if they are connected by roads not in a territory, chop them, or blockade the English Ports too.
Better, if you're at war with them, then chop the roards next to that city as well as blockading them. This will serve the same purpose. And if you chop off the roads around the captiol, say with a nuke or some such, then the resources that AI is trading will go bye-bye. Unless the captial is a port city, and has an active harbor.
Get's complicated pretty quickly, doesn't it?