Last month Sven Ortmann wrote an article on The Firearm Blog about small arms cartridges. His European perspective generated some controversy on the blog but it generated a great discussion. He has written a follow-on article on his blog. He told me that this article is merely an opinion piece, not intended as an essay.
Rifle calibers have been discussed and changed since we moved from musket ball to rifle bullets in the 19th century. The introduction of modern smokeless propellants brought another huge boost for caliber discussions, again toward smaller calibers. The final push toward smaller calibers (calibers smaller than 7mm were already tested at the beginning of the 20th century) came after WW2 when almost every soldier got a fully automatic weapon.