One of the threats to the U.S. economic boom is the collapse of various currencies and the possible collapse of banks in countries that are relatively large.  The 3 most visible problems that are in the headlines are Argentina, Turkey, and Venezuela.  A fourth that is emerging is Italy.  What do all of these have in common?  They have elected populist governments that are either on the left or right side of the political spectrum.  Why is this important to the U.S.?  Trump is a populist who ran a campaign centering on anti-immigration, economic protectionism and tearing down government institutions designed to protect individuals against organizations interested only in profits at any cost.  Other countries have also elected similar leadership including Hungary and Poland.  Russia and China of course have a history of strong armed leadership but the leaders of those nations are really not populists since they have control of the mechanisms that allow them to be in power.  They may have started out as populists however.   Now Putin is a quasi dictator and the leader of China is a reformed Communist with his own brand of Capitalism designed to keep the party of Mao in power. 
The real estate crisis and attacks on the banking system began showing their first signs of emerging in 2005 to 2006.  They originated in the U.S. but by the end of 2008 the banking crisis in the U.S. had spread around the world, liquidity everywhere was being challenged as everything was for sale, and only the quick financial  moves of Hank Paulson and Ben Bernanke prevented the complete collapse of the U.S. banking system and  the economy.  The next crisis maybe emerging outside of the United States.  Just one bad apple in the world economy would not be a threat to the U.S. economy–but many bad apples can lead to a panic and the associated liquidity problems.   
When the markets are making new highs it is like drinking the kool aid of self indulgence and creates a sense of false security.  People choose not to pay attention to what is wrong or showing signs of being wrong.  Complacency becomes the dominating force.  Carl