Most would say Russia but as China continues to increase its spending on its military and technology you can bet a day will come when the United States and China are at odds with each other. Taiwan is often thought of as that pending flashpoint where such would happen but as China continues to build it's islands into the South China Sea then claim territorial waters that it never originally had then that also could be a source of conflict and then there is always North Korea which China has promised to defend. People for a long time always thought Russia was going to be the most likely adversary we would get into a military conflict with but lately the great expansion of China and their super growth as an economic power puts China, in my opinion, as the greatest threat to the United States.
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