I believe that America as a nation is already on the decline due to the foundation of moral decay (abortions, homosexuality/transgenderism rights, divorce and remarriage, etc.) and a constitutional crisis between the Congress and the Presidency of Donald Trump. With all the infighting between Congress and the Presidency, and possible Supreme Court, they will never address or solve the nations's most pressing problems like Medicare, Social Security, rising drug costs, national infrastructure, national debt, budget, international trades, etc. America's standing in the world got so weaken that I won't be surprised if China will retake Taiwan sooner or later and bring much of East and Southeast Asia into her tighter sphere of influence. Turkey and Saudi Arabia are joining Iran in building nuclear weapons. Russia will continue to nibble at the borders of neighboring states which watching NATO losing its credibility with crisis in Eu and Briexit, etc. etc.
This is the decline of America, like any empire in history, however, that is NOT the reason why Christ will return. The real problem is with the Christian church. Some studies suggested that that America was about 90% Christianity decades ago but has "declined" to 70%. The problem is not 20% that we have lost over the decades, rather the problem is WITHIN the 70% that we still have! Why? There is something seriously wrong with our 70%. The 70% is not a small number, so the question is if 70% of use were really authentically substantially Christian, then our America culture, including our government and law, would still be in pretty fine shape. The sad fact is a GOOD PORTION of 70% Christians are not really authentically substantially Christian whatsoever. I think the reality is that the average American Christian is no different from the average American atheist or agnostic or secular. America is in fact, a secular country, and secularism is like disease that creeps into Christianity circles and infected (deceived) many people. That is why we are seeing more and more "Christians" of the churches who have forsakes God's Word by being more obsessed with prosperity, guns, nationalist and demeanor president, flag, national Israel, etc while buying their false prophets and christs' lying signs and wonders.