Which medieval trading league ran its own fleets, laws and embargoes across northern Europe?

The Hanseatic League - a confederation of merchant towns around the Baltic and North Sea, with Lübeck at its centre, that at its height involved close to two hundred cities and behaved like a state without being one.

It had no capital, no standing army, no written constitution and no fixed membership list; it had an assembly that met irregularly and could not compel anyone to attend. What it had was leverage. A town or a king who crossed it could be cut off from Baltic grain, timber, herring and wax, and the League fought and won a war against Denmark in the 1360s, imposing a treaty that gave it a veto over who became Danish king.

Its trading posts abroad were walled compounds under Hanseatic rather than local law - the Steelyard in London had its own quay, church and warehouses and lasted until Elizabeth I revoked its privileges. It faded as national states grew strong enough to regulate their own trade and as Atlantic routes drew commerce away from the Baltic. The last formal assembly met in 1669 and only three towns bothered to come.