to the international system in Western Europe, especially in Switzerland, Germany, Italy, and the Netherlands. Historian Frederick Kagan argues that Britain was irritated in particular by Napoleon's assertion of control over Switzerland. Furthermore, Britons felt insulted when Napoleon stated that their country deserved no voice in European affairs, even though King George III was an elector of the Holy Roman Empire. For its part, Russia decided that the intervention in Switzerland indicated that Napoleon was not looking toward a peaceful resolution of his differences with the other European powers. The British hastily enforced a naval blockade of France to starve it of resources. Napoleon responded with economic embargoes against Britain, and sought to eliminate Britain's Continental allies to break the coalitions arrayed against him. The so-called Continental System formed a league of armed neutrality to disrupt the blockade and enforce free trade with France. The British responded by capturing the Danish fleet, breaking up the league, and later secured dominance over the seas, allowing it to freely continue its strategy. But Napoleon won the War of the Third Coalition at Austerlitz, forcing the Austrian Empire out of the war and formally dissolving the Holy Roman Empire. Within months, Prussia declared war, triggering a War of the Fourth Coalition. This war ended disastrously for Prussia, defeated and occupied within 19 days of the beginning of the campaign. Napoleon subsequently defeated Russia at Friedland, creating powerful client states in Eastern Europe and ending the fourth coalition. Concurrently, the refusal of Portugal to commit to the Continental System, and Spain's failure to maintain it led to the Peninsular War and the outbreak of the War of the Fifth Coalition. The French occupied Spain and formed a Spanish client kingdom, ending the alliance between the two. Heavy British involvement in the Iberian Peninsula soon followed while a British effort to capture Antwerp failed. Napoleon oversaw the situation in Iberia, defeating the Spanish, and expelling the British from the Peninsula. Austria, keen to recover territory lost during the War of the Third Coalition, invaded France's client states in Eastern Europe. Napoleon defeated the fifth coalition at Wagram. Anger at British naval actions helped push the United States to declare war on Britain in the War of 1812, but it did not become an ally of France. Grievances over control of Poland, and Russia's withdrawal from the Continental System, led to Napoleon invading Russia in June