How to destroy your civilization

Simple.  Start World War I.  In 1913, the last year before the war, Europe ruled the world.  Her technology, steam railroad, telegraph, steamships, repeating firearms, telephone, machine woven textiles, electricity, and mass production was totally dominant.  Non European countries could not even duplicate European technology, they had to import it, from Europe.  The less advanced regions of the world were "colonized" (taken over) by European countries and run for the benefit of the colonizing countries.   
   After four years of slaughter and destruction on the Western Front, the Russian revolution, and the destruction of the Austro-Hungarian empire the European countries were too shattered, too broke, and too demoralized to keep it up.  What's worse, the seeds of the second world war, and the following  cold war had been planted.  In 1918 the Allies were too exhausted to invade and occupy defeated Germany, and convince the Germans that they had truly lost the war.  The Communists had seized power in Russia and would keep it for 70 years.  The majority of Germans felt they had not been beaten fair and square and were ready to try it again twenty years later.  This brought Adolf Hitler to power and kicked off WWII, which was as bad, or worse, than the first one. 
   What went wrong?  First.  The Austro-Hungarian empire could see and feel it's power crumbling in the face of nationalist feeling among it's massive subject peoples.  The German speaking Austrians and the Hungarians had struck a deal to share power and run the empire.  The rest of the empire, the Balkans, Czechs, Slovaks, Poles, Moldavians, Bohemians, and others were second class imperial citizens, and wanted out.  The ruling Austro-Hungarians knew that when the subject peoples got out, they would be reduced to running a third class eastern European now-wheres-ville.  They saw the Sarajevo assassinations as the pretext for a sharp little war that would  teach their subject peoples to shut up and do what they were told.  The ruling eleite believed that unless they took drastic action they were doomed, so they were strongly motivated toward war.   War was their salvation.
   Second.  Germany, a brand new country created just 45 years before, lacked national institutions with the power to constrain the central government, a monarchy with a nut case monarch.  The nut case liked international crises, the Sarajevo killings looked like a fine crisis, he decided to stir the pot.  When the Austrians came the Berlin asking for support in their hassle with the Serbs, he told them to go right ahead, kick some Serbian ass.  With that backing, the Crush-Serbia-Now faction in Vienna was able to silence their opponents and kick off the war.
   Lessons learned.  First, if you run an Empire, you want to give everyone in the empire a stake in it.  The Romans understood this; they would make anyone a citizen of Rome.  Even the Apostle Paul was a Roman citizen. The Austro-Hungarians might have survived and not needed a brisk little war to shore up the empire, if they had worked harder on giving everyone in the empire a fair shake.  Second,  you want to require assent from everyone in the country before going to war or taking steps that lead to war.  If  approval in the Reichstag and in the foreign ministry had been required for the infamous "blank check" that Wilhelm II issued to the Austro Hungarians, it would not have approved.  And WWI would have been avoided.