We have been picking on smaller nations for quite awhile,  Now we are taking on Russia and China. And at the same time! But we will have our Nato Allies with us.
Why do I say it’s already started? It started against China when they accepted promissory notes of payment for their cheap products. And now that we are unable to pay due to our massive debt and China is past being patient there is conflict. We are trying to work out land deals but that is hard to implement.

It started with Russia when they decided that having the dollar as the reserve currency is totally unfair (since that means all nations have to buy dollars in order to conduct trade). For instance a nation can only buy oil if they have US dollars. And we are on the opposite sides in the conflict in Syria.
WWII started when we put sanctions on Japan’s ability to conduct trade. Being an island nation they were very dependent on foreign goods. The attack on Pearl Harbor was wanted by the US govt. in order to get popular support for the war. By the way we made sure our valuable aircraft carriers were moved to San Diego and only disposable ships were left at the naval base at Pearl Harbor Hawaii.  We hoped Japan would see how vulnerable our “Pacific Fleet” was and try to destroy it. Which they did.