By Aaron Kliegman\u00a0 Hitler\u00a0may have been insane (at least strategically), but\u00a0even\u00a0he\u00a0could be deterred. And\u00a0indeed,\u00a0he was,\u00a0for a brief, instructive moment.\u00a0 The year was 1934.\u00a0Hitler\u00a0had been chancellor of Germany for 18 months and\u00a0coveted Austria, his birthplace. So,\u00a0the Nazi Party instigated a putsch to annex the land.\u00a0 Nazis assassinated the Austrian chancellor,\u00a0Engelbert Dollfuss,\u00a0attempting\u00a0to seize power.\u00a0In total, they\u00a0killed more than 100 people\u00a0and injured several hundred others.\u00a0 News of the\u00a0putsch\u00a0enraged Benito Mussolini, who was close to\u00a0Dollfuss\u00a0and not yet allied with Hitler.\u00a0In fact, the Italian leader was hosting Dollfuss\u2019s family at the time of the assassination. Mussolini had\u00a0also\u00a0declared strong support for Austria\u2019s sovereignty.\u00a0 So,\u00a0Mussolini moved the Italian army to the border between Italy and Austria, threatening\u00a0Hitler with war unless the Nazis stopped their plot.\u00a0 And guess what happened\u00a0next:\u00a0Hitler backed down. The putsch failed.\u00a0Mussolini of all people was the only one ever to deter Hitler.\u00a0 If only\u00a0other European leaders had showed resolve to stop Nazi Germany\u00a0before World War II\u00a0broke out in full.\u00a0 There were\u00a0certainly\u00a0several\u00a0opportunities to do so.\u00a0 In 1936, for example, Hitler ordered German troops to enter and remilitarize the Rhineland.\u00a0(The order\u00a0violated\u00a0the Treaty of Versailles, which had ended World War I.)\u00a0But Hitler also told his generals that, if France showed any signs of taking a military stand against the move, Germany\u00a0should\u00a0retreat.\u00a0 The French did\u00a0nothing, afraid to risk going to war.\u00a0 All\u00a0France\u00a0had to do\u00a0was demonstrate the\u00a0smallest\u00a0bit\u00a0of strength\u00a0to deter Hitler, who later\u00a0admitted\u00a0as much.\u00a0 \u201cA retreat on our part would have\u00a0spelled collapse,\u201d he said. \u201cThe 48 hours after the march into the Rhineland were the most nerve-racking in my life. If the French had then marched into the Rhineland, we would have had to withdraw with our tails between our legs.\u201d\u00a0 Hitler went on to explain that Germany\u2019s military was weak and would have crumbled in the face of even \u201cmoderate resistance.\u201d\u00a0 The same was true of the German\u00a0military\u00a0in 1938, when the Nazis finished what they started four years earlier and annexed Austria.\u00a0 In 1940,\u00a0after war erupted,\u00a0Hitler could have been\u00a0deterred\u00a0from invading France.\u00a0Even then,\u00a0Britain, France, and the US\u00a0had far more\u00a0military\u00a0assets than most people appreciate today. They\u00a0needed to\u00a0rearm\u00a0a little earlier and show a willingness to stop German aggression.\u00a0 Perhaps most striking, Hitler\u00a0suggested\u00a0during the war that he might\u00a0never\u00a0have\u00a0invaded the Soviet Union in 1941 had he known about the Soviet Red Army\u2019s T-34 battle tanks.\u00a0\u00a0 In short,\u00a0World War II possibly could have been avoided\u00a0\u2014 either through deterrence or, more likely and preferably, by destroying Nazi Germany when it was weak.\u00a0No, the world wouldn\u2019t have been a utopia, but maybe it could have been spared a global cataclysm.\u00a0 Why does this history matter today?\u00a0First, it proves\u00a0that even the most crazy, irrational, and ideological tyrants can be deterred\u00a0\u2014 that is,\u00a0if the other side signals its\u00a0capabilities\u00a0and a willingness to use\u00a0them. Second, it shows the tragedies that are possible if democratic countries don\u2019t invest the necessary money in their militaries and\u00a0are unwilling\u00a0to exert their power\u00a0early\u00a0against expansionist dictators.\u00a0 Stated differently, history demonstrates the folly\u00a0of appeasement.\u00a0 Consider Iran, whose leaders are deeply ideological and, like Hitler, despise Jews and seek the Anglosphere\u2019s demise.\u00a0 While Barack Obama was president, the Iranian regime never feared for a moment that the US would attack Iran. The Obama administration established no deterrence. Indeed, the administration made clear that it sought to\u00a0create\u00a0cordial relations with Tehran by making concessions, believing that, through more intimate ties with the West, Iran\u2019s leaders would moderate over time.\u00a0 That\u2019s how Iranian negotiators secured such an advantageous nuclear deal\u00a0for the regime, one that\u00a0paves\u00a0a path for Iran to obtain nuclear weapons.\u00a0At\u00a0the end of negotiations,\u00a0for example,\u00a0Tehran\u00a0demanded an end to the embargo on\u00a0shipments of\u00a0conventional weapons to\u00a0and from\u00a0Iran.\u00a0The US rolled over and\u00a0conceded, desperate\u00a0for a deal.\u00a0If the regime believed the US\u00a0was willing to use military force, it never would have\u00a0been\u00a0arrogant\u00a0enough to\u00a0seek such\u00a0a concession.\u00a0 If\u00a0Joe Biden\u00a0is elected president, he will\u00a0continue the Obama administration\u2019s posture toward Iran.\u00a0He and his top advisers have\u00a0said\u00a0a Biden administration would\u00a0seek\u00a0to re-enter the nuclear deal,\u00a0with the vision of creating a\u00a0more\u00a0peaceful\u00a0relationship between Iran and its neighbors \u2014 and between Iran and America.\u00a0 Biden would be wise to consider the words of Wang Xiyue, a\u00a0US\u00a0citizen and PhD candidate at Princeton who was imprisoned in Iran from August 2016 to December 2019. Wang\u00a0explains\u00a0in a new essay that, before his imprisonment, he thought Obama should visit Iran and move the bilateral relationship forward. But after witnessing the regime\u2019s \u201cactual workings from the inside\u201d for 40 months, his view changed.\u00a0 \u201cRapprochement is a fantasy,\u201d he writes. \u201cThe Islamic Republic thrives on tension with the United States.\u201d\u00a0 Indeed,\u00a0he adds,\u00a0the regime\u2019s \u201csurvival and its elite\u2019s prosperity require maintaining hostility\u201d with the US. That means concessions will only invite more aggression\u00a0and establishing cordial relations is impossible (at least with the current regime).\u00a0 Establishing deterrence, however, can contain Iran\u2019s belligerence. And Iran can certainly be deterred. What\u2019s required\u00a0is, above all, a\u00a0continued\u00a0robust investment in military power and a demonstrated willingness to use it.\u00a0The situation in the Middle East is far more complex, of course, but this is the broad foundation on which all else should rest.\u00a0 Such a posture doesn\u2019t reject\u00a0diplomacy but\u00a0strengthens\u00a0it. For negotiations to be genuinely fruitful,\u00a0Iran must know the US is willing to impose costs, with American power casting a shadow over the proceedings.\u00a0And no threat casts a larger shadow than\u00a0that of\u00a0military\u00a0force.\u00a0 The main problem with Biden, at least regarding Iran, is a lack of credible deterrence to undergird the rest of his policy.\u00a0 When the nuclear deal was finalized in 2015, critics\u00a0compared\u00a0Obama to Neville Chamberlain, invoking Britain\u2019s appeasement of Hitler. Chamberlain, the\u00a0British\u00a0prime minister, agreed to give Hitler part of Czechoslovakia\u00a0in 1938, hoping that would satiate Nazi Germany\u2019s appetite for war and prevent escalation. It didn\u2019t.\u00a0 But Chamberlain saw his choices as\u00a0either\u00a0appeasement or war with Germany. If Britain had rearmed by then and demonstrated a willingness to use its military, however, then the West would have had more and better options.\u00a0 As the events of 1934 make clear, the historical analogy goes well beyond the Munich Agreement.\u00a0Hitler showed not only the futility of appeasement with a committed enemy but also the efficacy of deterrence with one. Today, America has no more committed an enemy than Iran.\u00a0\u00a0 We should draw lessons from the past and apply them to the present.\u00a0Those who don\u2019t will repeat previous\u00a0mistakes. Joe Biden seems poised, if elected,\u00a0to\u00a0go down a familiar, disastrous\u00a0road.