Hello Amazing Grace: From what I can gather from the Bible; The USA will not factor in the endtimes. In fact if we are mentioned at all it would be as Young Lions of the Merchants of Tarsish. Either way, the young lions only ask (leaders of Gog/magog invasion force) if their intentions are more than for booty and spoils.
My supposition is that when the Rapture happens, it will cripple the USA as it has around 50-60% Christians. The economy will immediately tank. Also, many of these Christians are in the Armed services which will almost be depleted. Being a superpower and becoming a nation (in a twinkling of an eye) as weak as any of the third world country, will of course be conducive to attack/war taken to this nation by the enemies whose population only had a handful of Christians within their nations. China, NK, Russia, etc.
There will be those that are left behind, which I feel will be the Left leaning Liberals as most of them have thrown God to the dogs and the Me-Me-ME generation (Millennials). There are those people whose eschatology did not include the Rapture or those who listened to false teachings and embraced a soteriology that was incompatible with the Word of Jesus Christ.
It will not be a pretty mess; as after the Rapture happens, the Antichrist (White Horse (Seal #1)) will come on the scene. During the time that he is gathering his power base (as we are told that He will conquer with peace); the Red and Black Horse (Seals 2 and 3) will continue to produce Wars, Famine and Pestilence around the world.
And as they say;"the rest is History" as far as Revelation goes. Those left behind unless they are lucky, are going to DIE a horrible death. Some for Jesus Christ and others,,,,,,,, wellllllllll for Lucifer.