This phrase came into use in Roman times, when the Senate declared emperor Nero a " hostis publicus" in ad 68 ( Wikipedia). This was done to remove a threat to the republic. 
     Since then, it has become more sinister as a weapon of demagogues and dictators and those who cast a wide net to entrap anyone they see as an obstacle to their objectives.
     It was during the French revolution that its full force and its sinister purpose seen. 
     The words " ennemi du peuple" were extensively used . On Dec. 25, 1792 Robespierre stated " the revolutionary government owes to the good citizen all the protection of the nation; it owes nothing to the Enemies of the People but death"  ( Wikipedia), and many would die after so branded, even Robespierre himself. Included in the political crimes punishable by death were those " spreading false news to divide or trouble the people"( here you can glean the reason for Trump's use of the phrase, against the mainstream media and its reporters.) 
      In the Russia " vrag naroda" ( Wikipedia) was used by Lenin on Nov.28, 1917, calling for " enemies of the people" to be arrested and brought before the revolutionary court ( enemy of the laborers; enemy of the proletariat,  class enemy ).. This phrase was widely used in the Soviet Union and hundred of thousands were killed or sent to the " gulags".
 Mao used it in 1957, against  " social forces and groups that resist the socialist revolution" and hundreds of thousands died or were " reeducated"
     Nazi Germany used it in its plan to RELOCATE Jews to Madagascar " in former times sane people and sane leaders of the peoples made short shrift of the Enemies of the People. They had them either expelled or killed" ( Wikipedia) ( millions would be relocated and murdered).
     In the UK , in the aftermath of the referendum on membership of the EU, the Daily Mail, described judges who ruled that leaving would require the consent of the British Parliament, " Enemies of the People".
    In the United States, Trump has picked the usage, " the FAKE NEWS media ( nytimes, abc, cbs, cnn, nbc) is not my enemy, it is the enemy of the American people". He has been beating this drum incessantly . Each day that goes by is replete with tweets about the " fake news " and " enemies of the people". It is self serving, dangerous and a threat . He does this to deflect and detract from the reporting on the various investigations on Russian hacking; on possible collusion; on his various relationships with women, who were paid to be silent on their affairs with him; on the people leaving his Administration; on those closely connected to him, who have been found guilty of various crimes and to rouse up and keep his " base'/ of support.
     This phrase " enemy of the people" is once again being used to perpetuate divisions; to further ones agenda, either to wrest power or to hold on to power or to be a tool to cast doubts about crimes that may have been committed, or to dull attempts to expose wrongs. It has been used in the Spanish Inquisition; in the Salem " witch trials"; in the French Revolution; in the MC  Carthy " witch hunt"; in Nixon's "Watergate' and now by Trump. In all these cases its use has been iniquitous,  perfidious and malicious.    

     Trump's decision to place  punishing tariffs on Turkey is having a very unpredictable effect on an already unpredictable and unstable region. The Turks have decided to fight back and strongman Erdogan ( just re-elected) will not back down and will move Turkey closer to its neighbor but long time " threat" Russia. Already overtures have been made and Putin is willing and eager to step in and pull the long time N A T O member and the guardian of its Eastern flank, ( where many U S A military bases are located and from where many U S-led missions to Iraq, Syria etc. were launched), away from Europe and the military alliance. 
     Turkey as signaled its intention not to cave, by putting its own tariffs on American products and to take a loan of $15 billion from Qatar ( which has had a running economic battle with Saudi Arabia. Trump supports the Saudis in its row with Qatar ( an ally, but I guess the Saudi wealth and oil and the gaudy welcome have won Trump to their side). So here we have a NATO member being flayed economically by the ostensible leader of the alliance, turning for help from Russia and getting economic aid from Qatar ( Trump has weakened NATO and this will not help).
     The Saudis continue their genocidal war in Yemen, where they indiscriminately target civilians ( the latest being the busload of young students), with the assistance of US and Britain. Why? Oil and investments from the wealthy Saudis. They have also targeted ( as noted above)  Qatar ( even their camel herd were deported) and has picked a fight with Canada over Canada's request the women rights activist Badawi ( her blogger brother is presently imprisoned ). 
     The Saudis became furious ( they were afraid that others would join Canada and expose their woeful record on human rights). They severed diplomatic relation, asking the Canadian ambassador to leave; placed a hold on all new investments; cancelled all flights by Saudi airline and recalled thousands of students on scholarship in Canada. The Trudeau government has so far stood firm and has not even cancelled the $15 billion arms deal ( that was signed by the previous Harper government). Relations are tense and Canada's longtime allies US and GB have been silent ( I guess Human Rights is not as important as Saudi wealth, so they oppress their people, deny women rights, crucify and display  body them openly, carry out a genocidal war and the " democracies" remain silent.
     Canada and Turkey are both NATO members but in Trump's world that counts for little ( Trump has also placed punitive tariffs on many Canadian goods/products, even though we have an economic pact, NAFTA. 
     In the Middle East, Russia rules while US picks fights with Turkey and cancelled the Iranian nuclear agreement ( against the wishes of the other signatories and even though his own officials agree that Iran has not broken the agreement. Syria with the help of Russia and Iran is winding down its civil war ( which has killed thousands and driven tens of thousands into  forced exile). Iraq continues to be a battleground and so too Libya and  in Yemen the Saudi-led, US -backed coalition continues to decimate the population. Israel launches attacks on Gaza and even in Syria ( recently accused of assassinating a top Syrian scientist and targeting Iranian positions). 
     The region is a sizzling cauldron and the next eruption is not far off. It will be far ranging