Once upon a time... it all started in the 1670's when England and France had one of their spats which led to punitive import and export duties. Now, because many of the English back then were sophisticated lushes, the decreased wine from France really hurt the English. So the thirsty English went further south to find wine to make up the difference; to Portugal. The British found the oak barrels of wine exploding in the hulls of the merchant ships on the voyage to England. Here's the apocryphal short story: A couple of English merchants walked around Portugal to figure out a solution to their exploding barrels. So two young British merchants, maybe seeking divine inspiration to their dilemma, found themselves at a monastery outside the town of Lamego, near the Douro River in Portugal. The abbot there served them a wine that was smoother, sweeter, and more interesting than any they'd tasted. When the young men pressed the abbot to tell them more, he confided that he'd added brandy to the wine as it fermented.
So how is Port made?
Here's a simplified version of a complicated process: Add one part grape spirits (clear brandy, 150 proof) to four parts red wine while it is fermenting. The brandy is added when about half of the natural sugar has been converted to stop fermentation. The alcohol in the brandy causes the yeasts in the wine to die, stopping fermentation. The result is a sweet wine with about 10% residual sugar, fortified to just under 20% alcohol.