Explain the general characteristics of fascism, why it always has a nationalistic background, and any socioeconomic variables that are typical of the demographic of supporters. What was the threat posed to Western-style democracy in the US, Europe, and Asia by fascist regimes?

Fascism, by definition, is a dictatorial form of government based largely on exalting the importance of the "nation" above the freedom of the individual. Therefore, to ask why fascism has a nationalistic background is basically to ask why the movement existed in the first place and why it came into being as it did, during the first half of the twentieth century.
Nationalism and ethnic self-determination were forces that had dominated politics and the public consciousness of the nineteenth-century world. In the aftermath of the Great War (World War I) in the following century, these forces took an even more extreme form, partly because certain peoples believed they had been treated unfairly in the settlement that ended the war and partly because they wished to recreate a perceived past glory that had been lost or allegedly taken from them. Right-wing elements in Germany were infuriated over the huge war reparations the Germans had been made to pay and the economic depression that occurred as a result. They also wished to recover territories previously controlled by German-speaking people that had been made into newly independent countries, such as Czechoslovakia. Hitler and his Nazi Party blamed the negative outcome of the war and the settlement after it upon a supposed "Jewish conspiracy." The Nazi form of fascism was thus racist as well as chauvinistic. In Italy, Mussolini and his followers had the goal of recreating the Roman Empire. In Japan, the extremists believed in the superiority of the Japanese over the other Asian nationalities and had the goal of both creating an Asiatic empire and of expelling the European colonial powers from Asia.
These manifestations of fascism were both an intensified (and even perverted) form of the long-standing cultural trend of nationalism and a reaction, to an extent, to the specific circumstances of the period after World War I. They were also a reaction against the threat of communism, or were rationalized as such. In Germany, Italy, and Spain (which became a fascist state after the Spanish Civil War of the 1930s), many people supported the fascists because they considered them the lesser of two evils, fearing the communists even more.
The fascist states in both Europe and Asia were a threat to the democratic countries because the fascists had expansionist aims. As stated, the Japanese had the goal of creating a vast Asian empire by subjugating China and the smaller nations and by expelling the Europeans from Asia. The Germans intended to exile or exterminate the Jewish population of Europe and to expand Germany by taking over the Slavic nations to the east and subjugating their populations. Italy, though not explicitly racist as the Nazis were, had, as stated, the intention of creating a new Roman Empire by taking over Greece and parts of Africa as well. World War II thus became one of the most genuinely "necessary " wars in history, and with the victory of the Allies in 1945, the threat of fascism appeared to be over.