China would be the biggest worry. The Russian military is still only a shell of what it was 30 years ago after communism there declared itself bankrupt and decided it needed to become more capitalist. Russia only has an economic strength of about a 10th or less of the US economy by comparison, and the North Korean economy is worth about 2 bags of rice, which leaves China which has an economy not much less than the United States. It is very unlikely China would want a war with the USA as the US would stop importing all their goods and China's economy would collapse. North Korea has already be warned if it starts a war with its stupid talk threatening the US and South Korea with nuclear strikes China and Russia will not support them so North Korea knows it does not have the ability to destroy a large force of US and allied military. You Americans seem to not realise that the US does not fight alone and countries like Australia, Britain, Germany and many other nations fight with the Americans but the US media only likes to report on what their own military are doing and forget to mention all the other nations there fighting with them. No country can afford a war fighting Russia, China or the US and they will do everything they can to avoid it.
Next time you go shopping check where the item you are planning to buy is made. If it is made in China and you buy it money goes back to China which allows them to build a bigger military as well as cause US factories close down as they cannot compete on price, putting Americans out of work, making America poorer and China richer and stronger. If China did start a war with the US basically your purchases would have helped them buy and make their military weapons American soldiers would be killed by. Think about it when you buy a Chinese product as generally Chinese products are not well made compared to the local product and so not as good to buy even if they do cost less. America buy locally made goods or watch China take over as the strongest nation in the world while America slips backwards towards being a 3rd world country.