Continental System , a plan by which Napoleon I of France hoped to defeat Great Britain by shutting off its sea trade. In 1806 Napoleon issued the Berlin Decree, announcing a blockade of the British Isles and closing all the ports of Europe to British trade. England replied in 1807 by issuing Orders in Council providing for blockading ports of all countries observing the Berlin Decree. Napoleon then issued the more drastic Milan Decree, stating that neutral vessels sailing in or out of British ports would be subject to seizure.
Napoleon's land power was so great that all European countries except Great Britain and the Ottoman Empire eventually belonged to the Continental System. (Russia, however, was strong enough to withdraw in 1810.) The French navy was too weak to enforce the blockade on the seas. Great Britain's superior naval power enabled it not only to run large quantities of goods through the blockade, but also to cut off a great amount of French shipping. Portugal persisted in trading with Great Britain, and Napoleon seized both Portugal and Spain to strengthen the system. That action, however, failed to make the Continental System effective.