Symphony D.
Deity
Trade Route: General path along which trade flows.There is a significant difference between trade routes, trade agreements, and tarriffs. Switch your subject, and certain elements of your argument will become invalid.
Trade Agreement: General or specific agreement to facilitate trade by lowering tariffs.
Tariff: Taxation upon foreign goods.
Tariffs only really work at excluding items from the market if you have them to begin with. The existence of a trade route is more or less exclusively as a result of foreign luxury items not to be found at home, eg: silk, sugar, etc. Some of that is covered by colonies. Some is not. If you have no way of producing a good but want it anyway, obviously, you are going to have to get it from elsewhere. Ta-da, the trade route is born. It runs through several nations, does not consist of official government convoys typically, and is only occasionally protected by government forces from the predation of brigands and thieves. It is not built or conducted by either the governments at the origin or the destination of those goods typically. Colonies are an exception, but again, as previously said, colonies are not international trade, and are not involved in this discussion.
You can play with the semantics all you want, the fact is long-distance luxury trade--the kind that produces trade routes--is not dictated by government-to-government exchanges on a day-to-day basis at this stage in history, and, as I have repeatedly said, should therefore not be in their capability to execute.