Stuff flows downhill, swiftly

Lessons from history.  Hitler came to power in Germany in 1933.  In 1936 he re occupies the Rhineland.  This was a stretch of German territory bordering Belgium and Holland.  The treaty of Versailles, which ended WWI, declared that the Rhineland must be demilitarized, no soldiers, fortifications, no so much as a foxhole.  A mere three years after taking power, Hitler felt strong enough to defy the Versailles treaty and send the German army into the Rhineland. 
   What should have happened,  didn't.  The British and the French had ten times Hitler's troop strength in 1936.  They should have moved troops into the Rhineland, arrested or shot any German in uniform, and restored order, the Versailles way.  If the Americans had supported the British and French (which we did not) it would have worked.  Hitler would have lost enormous amounts of prestige, and might well have lost his office and his life.  And that would have prevented WWII.  But we did nothing, the British and the French did nothing, and WWII broke out 3 years later.
   In 1938, Hitler "absorbed" Austria. A sizable territorial gain, Austria is maybe 10% the size of Grmany, kind of like California is to the rest of the United States.  Austria was the Germany speaking part of the old Austro Hungarian empire, the part that used to run said empire.  They had been trimmed back from Great Power status to third class European power status and they wanted to join the successful Nazi juggernaut, rather than be a footnote to history.  With this kind of popular support for Anschluss, there wasn't much anyone could have done.  But if the Rhineland reoccupation had been crushed two years before, the Anschluss probably would not have happened. 
  Later the same year, Hitler demanded the German speaking parts of Czechoslovakia be turned over to him.  Hitler threatened war if he didn't get his way.  The British and the French came to the infamous Munich conference, and joined hands with Hitler in browbeating the Czechoslovakians into yielding to Hitler's demands.  The Americans stayed out of Munich.  What should have happened.  The British and the French declare war on Germany then and there and launch an invasion thru Germany's western border.  This would have been trickier than the Rhineland, 'cause the German army was a lot stronger in 1938 than in 1936.  But it wasn't yet strong enough to grab Czechoslovakia and fend off an Anglo French invasion at the same time. 
  Next year, 1939, Hitler managed to start up a Czechoslovakia Nazi party and take over the rest of the country by "legal" political subversion.  By summer of 1939 Hitler decided to take over Poland.  To his surprise, the British and the French had grown a pair between them and told the Poles they were 1000% behind them.  Poland refused Hitler's demands. Hitler invaded in September of 1939, France and  Britain declared war on Germany and WWII was off and running. 
   Moral of the story.  Let a bad guy get away with just one little thing, and the whole world can slide down the tubes. 
   So far we have let Assad stay in power in Syria, let Libya dissolve into chaos, pulled our troops out of Iraq  letting ISIS take over, let Putin grab big chunks of Ukraine. For our next smooth move we pull our troops out of Afghanistan.   That's more than just one little thing.