Why are the Americans so confident in there military?

I was reading a statistic saying if the Americans go to war with the North Koreans they Would lose 250 000 people. Now just taking a guess you would triple it to 750 000 wounded with makes it 1 000 000 causalities. North Koreans with old iron wooded rifles with old rusty ranks and cheap uniforms with a tin helmet. How can Americans suffer this much from a bunch of Barbarians? :O Every forum I been on Americans say they can bring China, North Korea, Russia to its knees in 6 months to a year and destroy everything in their military. Its funny how they think there so good and get there *** kicked. Look at the Eastern Front in World War II. Germans were seen as the best, lots of men, best weapons, best tactics, best strategies, best modern everything, and they still lost to the Soviet Union. The Soviet army back then was in much worse shape then the Russian army is now. So what makes the Americans think they will win against Russia? Look at North Korean statistics, it will be much much worse with China or Russia.