<P> The Continental System or Continental Blockade (known in French as Blocus continental) was the foreign policy of Napoleon I of France against the United Kingdom during the Napoleonic Wars . As a response to the naval blockade of the French coasts enacted by the British government on 16 May 1806, Napoleon issued the Berlin Decree on 21 November 1806, which brought into effect a large - scale embargo against British trade . The embargo was effective intermittently . It ended on 11 April 1814 after Napoleon's first abdication . In terms of economic damage to the UK, the blockade was largely ineffective; however, British exportations to the continent saw a loss from 25% to 55% of their total value in between the years 1802 and 1806 . As Napoleon realized that extensive trade was going through Spain and Russia, he invaded those two countries . His forces were tied down in Spain--in which the Spanish War of Independence was occurring simultaneously--and suffered severely in, and ultimately retreated from, Russia in 1812 . </P> <P> The Berlin Decree forbade the import of British goods into European countries allied with or dependent upon France, and installed the Continental System in Europe . All connections were to be cut, even the mail . British merchants smuggled in many goods and the Continental System was not a powerful weapon of economic war . There was some damage to British trade, especially in 1808 and 1811, but its control of the oceans led to replacement trade with North and South America, as well as large scale smuggling in Europe . The loss of Britain as a trading partner also hit the economies of France and its allies . Angry governments gained an incentive to ignore the Continental System, which led to the weakening of Napoleon's coalition . </P>

Why did the british block the us ports from trading with france