The Weimar Republic failed for myriad reasons, but I would argue that there are three key themes:

• Economics: The Republic was undermined by crippling financial problems, for which it seemed to have no answer.
• Politics: Republican ideals had always rested with the Social Democrats, and the Republic's very existence was intrinsically linked to the SPD. When the SPD lost traction and popularity in the late 1920s, the political forces now responsible for upholding the Republic's integrity were political parties that were hostile to it.
• Psychology: All this fed into a public sentiment that was battered and bruised after the privations of the First World War, leading to an unpredictable situation that ultimately resulted in the collapse of the system.

It is often said that the Weimar Republic was doomed from the very beginning, when the fledgling government was compelled to agree to the Treaty of Versailles and thereby condemn Germany to an unjust peace. This is fallacious for a couple of reasons. First, the Treaty was actually far more lenient than is often supposed. John Maynard Keynes believed that the terrible strain of admittedly outrageous reparations would force Germany into humanitarian crisis; Vladimir Lenin believed that it could only result in the Germans rising as an angry mass after they had been beaten down. In fact, Germany never really felt too much of the bite of reparations, as a result of a series of skilful manoeuvres in the early 1920s on behalf of successive SPD-led or -backed governments, and in particular the erstwhile foreign minister Gustav Stresemann. It was Stresemann who conducted negotiations with the chief economist of the Dawes Committee, Owen Young. Young's job was to determine the rate at which the Germans would pay reparations to the victorious Allies. Young, however, had a conflict of interest. In addition to being attached to the committee, he was also the chairman of General Electric, which had just entered into a commercial agreement with the German electrical firm AEG. Young reasoned that American investment in Germany would provide an economic stimulus to the country, which would in turn increase demand on the market to which AEG belonged, which would end up benefitting General Electric. In order to achieve this, Stresemann and Young came up with an agreement whereby the merchant banker J.P. Morgan would extend large high-interest, low-security loans to German banks. What this meant was, while in the immediate aftermath of the war the German economy struggled, very soon afterwards it began to enter a new era of prosperity, propped up by American investments. This also meant that Stresemann had come up with a fairly ingenious way of sidestepping the requirements of the Versailles Treaty. Germany received American money, then used some of that money to pay its debts to France and Britain, which in turn used the same money to pay their debts to the United States. It was also in this context that the Weimar Republic soon became the byword for artistic avant-garde sophistication. Berlin, in particular, became famous (or, perhaps, infamous) for its hedonistic clubs and cabarets. It soon became chic for much of European high society and the aspirational classes to be seen in Berlin, taking advantage of the dancing, the alcohol, the unconventional art scene, and also the ubiquitous prostitution.

Yet this newfound prosperity was built on a house of cards. By 1925 Morgan's loans (and those that followed from other banks) formed a sort of symbiosis; the Republic could not survive without American money. Stresemann's gamble was that the German economy would be back on its feet and functioning before the Americans recalled their loans, but when Wall Street suffered its stock market crash in 1929, leading to a run on American banks, those banks responded by withdrawing their funds from overseas investments. This effectively pulled the rug out from under the economic miracle of Weimar. Virtually overnight, German banking collapsed and this, in turn, proved catastrophic for the businesses that had benefitted from the banking stimulus. Immediately before the Crash, 20 million Germans were in paid employment; by 1933 just 11.5 million drew a wage, though even this is inflated by seasonal work. In addition, social welfare, already stretched before 1929, could hardly cope with the sudden influx of mouths to feed and rents to pay.

The collapse of the German economy also proved to be too much for the SPD. It was still the largest political party in Germany, but support had eroded as a succession of governments had been indecisive and incompetent. The end of the few years of prosperity exposed the SPD's plan of recovery as little more than a sham. Its loss of face became a loss of position, and it was effectively replaced in government by a succession of right-leaning parties, each more nationalistic and authoritarian than the last. Indeed, these nationalists often hailed from the old school of Prussian conservatives, formerly loyal to the imperial Hohenzollern monarchy; among these we should probably count the president of the Republic, the former field marshall Paul von Hindenburg. These politicians had been suspicious of the "socialist" Republic from the very outset. The irony was that, with the actual socialists (the SPD) in decline, the levers of control of the Republic now fell to the same people who despised what the Republic stood for.

Finally, though, we cannot discount the German people themselves. There are old historical tropes that suggest that the Germans were "unready for democracy", but this is not the case; indeed, even before the war, German liberal democracy, while not yet at the same sort of representative level as Britain's, was strong and developing. But the Germans had been through a lot. The loss of the war, coupled with a fatalistic war-weariness, had been bad enough. Hunger and poverty had set in immediately afterwards. Then, they had experienced prosperity, only for it to be snatched away from them again. Under these circumstances, it was natural for them to seek alternatives and, with the left-wing SPD having largely discredited itself, those alternatives seemed more likely to exist on the Right. In particular, the NSDAP of Adolf Hitler, though little more than a fringe party before the Crash, now came to prominence through skilful use of rhetoric and propaganda. Nazi condemnations of social democracy merged with warped cultural communal memory. Now, Weimar's legacy of cultural experimentalism and a "golden age" became a legacy of depravity and lawlessness. Much has been written of a number of serial killers who sprang up during the Weimar years, such as Fritz Haarmann, the "Werewolf of Hannover", who would rape and murder young boys by biting into their necks. Haarmann's case became more and more a symbol of Weimar's perceived immorality and debauchery, exacerbated by the culture of cabaret and prostitution. Alfred Döblin's Berlin Alexanderplatz (1929), arguably the quintessential German novel of the twentieth century, captures much of the feeling of doom as Weimar spiralled to destruction: the protagonist, Franz Biberkopf, is released from prison, having served his prison sentence for murdering his lover. Upon his release, he befriends a prostitute, but she is in turn murdered by his one-time mentor. Perhaps no book better explains why the Republic, which started with some promise, ended largely unlamented.

The end, really, came in March 1933, when the newly-appointed chancellor, Adolf Hitler, was able to pass the Enabling Act, a law which effectively allowed Hitler and his cabinet to govern without consulting the Reichstag. By this time, though, the Republic had become little more than a sham. The nationalist governments of Brüning and Papen had given way to Hitler but, while Hitler's of course became the genocidal war machine that Brüning's never would have, in terms of their devotion (or lack thereof) to democratic republicanism they were effectively birds of a feather. So, while the official end to Weimar came with the Enabling Act, it had begun its terminal spiral almost from the moment the stock market crashed on the other side of the Atlantic.

Sources

Bracher, Karl Dietrich. The German Dictatorship: The Origins, Structure, and Consequences of National Socialism. Harmondsworth. Penguin. 1978.

Burleigh, Michael. Sacred Causes: Religion and Politics from the European Dictators to Al Qaeda. London. Harper Perennial. 2006.

Evans, Richard J. The Coming of the Third Reich. London. Allen Lane. 2003.

Evans, Richard J. The Third Reich in Power, 1933-1939. New York. Penguin. 2005.

Evans, Richard J. The Third Reich at War: How the Nazis Led Germany from Conquest to Disaster. London. Allen Lane. 2008.

Kershaw, Ian. Hitler, 1889-1936: Hubris. New York. W.W. Norton & Company. 1999.

Kershaw, Ian. Hitler, 1937-1945: Nemesis. New York. W.W. Norton & Company. 1999.

Tooze, Adam. The Wages of Destruction: The Making and Breaking of the Nazi Economy. New York. Penguin. 2006.

I'd also recommend Detlev Peukert and Eric D. Weitz on Weimar, but I don't have the citations handy, sorry!