Since this is a groundbreaking game with an eye on historical accuracy and originality, I will recommend all kinds of historical magazines. The ones most commendable would be those of greatest popularity in parts of the world where you already have good sales forecasts for your games. HOI3 will have a huge potential, and could achieve popularity among all age groups. It will never reach the great masses if the advertisement and reviews only occur on the Internet-based gaming sites though, where so many already are familiar with your concept.
Yes, good thinking, as I am often working from Madrid, there are many very good historical magazines there.
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Historia y vida, this is a GREAT Spanish historical magazine, with always WWII articles, sometimes there are commercials about wargames inside, see
http://www.prismapublicaciones.com/detalle_comercial.php?id=18&tipo=1 (if any help is required for the Spanish, let me know)
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National Geografic Historia Well know magazine linked to the network, see
http://www.historiang.com/
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La aventura de la Historia, from the Spanish newspaper El Mundo, see
http://www.elmundo.es/ladh/
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Clio Historia, website is under revision, but is large in all of Europe and maybe even South America, I think it originantes from Catalunia. See
http://www.cliorevista.com/