While grapes have been grown in Portugal over 2000 years ago - the Romans who arrived in Portugal in the 2nd century and remained over 500 years, grew vines and made wine along the Douro river, where Port is made today - wine only became an important export when the Kingdom of Portugal was established in 1143. However, Port wine, as we know it today, occurred much later - in the second half of the 17th Century. In 1386, a close tie was formed between Portugal and England (The Treaty of Windsor) which allowed active trading links between the two countries and many English merchants settled in Portugal. Several became wine makers. In the 1550's, significant amounts of Portugese wines were exported to England in exchange for salted cod. ( bacalhau, in Portugese - where it eventually became the national dish of that country). In 1654, special privileges and preferential customs duties were allowed for the English and Scottish merchants in Portugal to import wool and cotton from England and to export a number of Portugese products, including what was called. "Red Portugal", a light, crisp red Vinho Verde produced in the Menho region of Portugal,  The "Red Portugese" wine was not as popular in England as wines from France. In 1678, however,  England and France were at war and the English fleet blockaded French ports creating a wine shortage in England. In 1703 the English and Portugese signed the Methuen Treaty which, among other things, brought about the trade of English cloth for Portugese wines. The increased demand for Portugese wine, with its potential for higher profits, caused the English wine producers to explore and grow grapes further inland in Portugal. By doing this, they found that wines made from grapes grown in the area of the Douro River, with the town of Oporto at it's mouth, were richer and more palatable. These wines took the name of the city of Oporto, from which they were shipped, becoming known, in Portugese, as "Vinho de Porto", meaning wine from Oporto, and, in English, as Port". However, these wines were not the  fortified, rich, sweet. dessert wines that we know as Port  today. These evolved by accident. The evolution began when a small amounts of brandy were added to the wine as a preservative for shipment.The addition of the brandy stopped the continued  fermentative transformation of the sugars in the wine to alcohol. In 1820, a vintage occurred with a very high sugar content. The addition of brandy to this wine vintage stopped the fermentation process leaving a brandy "fortified" wine higher in sugar content with a semi - sweet taste that was embraced, immediately, by the British. Because of the popularity of this "new" kind of port wine in  England, the expatriate, British wine  makers (and a few others) in Portugal cultivated their wine grapes to contain higher amounts of sugar to continue to make their sweet, brandy - fortified Port wines.  From England, Port found it's way around the world. While "Port" type wines are produced in countries other than Portugal today, the only "true" Port comes from Portugal and many of the world's most famous Port wine houses remain British.