Question: When did the U.S. stop trading with the country that started hostilities?

Knowledge 1: Pacific War
It is generally considered that the Pacific War began on 7/8 December 1941, on which date Japan invaded Thailand and attacked the British possessions of Malaya, Singapore, and Hong Kong as well as the United States military bases in Hawaii, Wake Island, Guam and the Philippines. Some historians contend that the conflict in Asia can be dated back to 7 July 1937 with the beginning of the Second Sino-Japanese War between the Empire of Japan and the Republic of China, or possibly 19 September 1931, beginning with the Japanese invasion of Manchuria. However, it is more widely accepted that the Pacific War itself started in early December 1941, with the Sino-Japanese War then becoming part of it as a theater of the greater World War II.[nb 9] [SEP] Knowledge 2: Events leading to the attack on Pearl Harbor
Beginning in 1938, the U.S. adopted a succession of increasingly restrictive trade restrictions with Japan. This included terminating its 1911 commercial treaty with Japan in 1939, further tightened by the Export Control Act of 1940. These efforts failed to deter Japan from continuing its war in China, or from signing the Tripartite Pact in 1940 with Nazi Germany and Fascist Italy, officially forming the Axis Powers.
Knowledge 1: British Empire
The Caribbean initially provided England's most important and lucrative colonies, but not before several attempts at colonisation failed. An attempt to establish a colony in Guiana in 1604 lasted only two years, and failed in its main objective to find gold deposits. Colonies in St Lucia (1605) and Grenada (1609) also rapidly folded, but settlements were successfully established in St. Kitts (1624), Barbados (1627) and Nevis (1628). The colonies soon adopted the system of sugar plantations successfully used by the Portuguese in Brazil, which depended on slave labour, and—at first—Dutch ships, to sell the slaves and buy the sugar. To ensure that the increasingly healthy profits of this trade remained in English hands, Parliament decreed in 1651 that only English ships would be able to ply their trade in English colonies. This led to hostilities with the United Dutch Provinces—a series of Anglo-Dutch Wars—which would eventually strengthen England's position in the Americas at the expense of the Dutch. In 1655, England annexed the island of Jamaica from the Spanish, and in 1666 succeeded in colonising the Bahamas. [SEP] Knowledge 2: Events leading to the attack on Pearl Harbor
Beginning in 1938, the U.S. adopted a succession of increasingly restrictive trade restrictions with Japan. This included terminating its 1911 commercial treaty with Japan in 1939, further tightened by the Export Control Act of 1940. These efforts failed to deter Japan from continuing its war in China, or from signing the Tripartite Pact in 1940 with Nazi Germany and Fascist Italy, officially forming the Axis Powers.
Knowledge 1: British Empire
At the end of the 16th century, England and the Netherlands began to challenge Portugal's monopoly of trade with Asia, forming private joint-stock companies to finance the voyages—the English, later British, East India Company and the Dutch East India Company, chartered in 1600 and 1602 respectively. The primary aim of these companies was to tap into the lucrative spice trade, an effort focused mainly on two regions; the East Indies archipelago, and an important hub in the trade network, India. There, they competed for trade supremacy with Portugal and with each other. Although England ultimately eclipsed the Netherlands as a colonial power, in the short term the Netherlands' more advanced financial system and the three Anglo-Dutch Wars of the 17th century left it with a stronger position in Asia. Hostilities ceased after the Glorious Revolution of 1688 when the Dutch William of Orange ascended the English throne, bringing peace between the Netherlands and England. A deal between the two nations left the spice trade of the East Indies archipelago to the Netherlands and the textiles industry of India to England, but textiles soon overtook spices in terms of profitability, and by 1720, in terms of sales, the British company had overtaken the Dutch. [SEP] Knowledge 2: Declaration of war by the United States
The last time the United States declared war on any nation was in 1942, when war was declared against Axis - allied Hungary, Bulgaria, and Romania, because President Franklin Roosevelt thought it was improper to engage in hostilities against a country without a declaration of war. Since then, every American president has used military force without a declaration of war.

Answer: 1940