Fun fact: in both world wars, Germany was absolutely dependent upon Swedish iron ore to produce high-quality steel. In WWI, it was only the High Seas Fleet of battleships and battlecruisers that prevented the Royal Navy from sailing into the Baltic and choking off this supply and ending the war (an under-appreciated reason for Germany building all those capital ships in the first place while being generally unwilling to risk losing them all in a major fleet engagement). In WWII, it was Hitler's quick occupation of Norway and the Luftwaffe that preserved the supply of Swedish ore.

Germany was also critically dependent upon Swedish ball-bearings, but at least Sweden sold those to both sides.

That may have been part of it, but another part is that there was relative peace and stability for about a century, many government reforms that modernized the country, and the industrial revolution making Germany into a world power. Also, AFAIK, iron was relatively abundant in the Ruhr valley, along with coal, which is why it became the industrial centre for the country.