The modern British style of Porter has elements of ale-like fruitiness with a restrained level of roasted bitterness, ending with a prominent malt character and dabs of toffee, caramel and chocolate. Its revival in the UK can be traced to Penrhos Court brewery in Herefordshire in 1978, the first recipes relying mainly on digging out memories and a few brewery records. Some divergence between lighter (4.0-5.0% ABV) and more robust versions (5.1-6.2% ABV) is now occurring.
London brewers began exporting strong porter to the Baltic port of Danzig (now Gdánsk) in the 18th century, prompting brewers in Sweden, Finland, Poland and the Baltic states to start mimicking the style with varying degrees of success. The Finnish version has always been top-fermented, as was the Polish until well into the 20th century. From 1870 however bottom-fermentation became the norm in Poland and the Baltic States. Modern versions (6.5-9.5% ABV) are rich and heavily caramelized with dabs of roasty bitterness. Danish variants, at the lighter end, are sometimes termed Stout.
Porter was certainly made in the US, particularly on its East Coast, prior to Prohibition, and maybe a lot longer. It was revived in the early 1970s by Anchor Steam brewery in San Francisco. Its modern version owes much to the enthusiasm for ‘more’, in this case meaning more grain, more brown malts than black, and more aroma hops, ending a tad stronger than others (4.8-6.5% ABV).
Every now and then a brewer does something simple and creates a modern classic. It this case it was the Alaskan Brewing Company, where a fraction of smoked malt was added to their Porter experimentally in 1988. Since then as interest in craft beers has spread and groups of enthusiasts start testing out their palates, this unusual style (5.5-6.5% ABV) will sidle in as if it had always been there. Whether smokiness was a feature of 18th and 19th century porters is far less clear.