Tango has its roots in the late-nineteenth century South America, particularly the barrios of Buenos Aires, Argentina, where it found prominence among lower-class citizens. It evolved as a mix between the traditional, highly-technical flamenco dancing of Spain and faster, sensual, more carefree milonga dancing. During this time, tango was largely overlooked by historians, academics, and serious dance aficionados.
However, its popularity grew over the next 40 years or so, and by the time World War I had come around, it had lost its stigma as a pastime for the dredges of society. Wealthier castes and societal elites in various parts of the world, particularly Western Europe where it was thought to be much more exotic, not only embraced it, but turned it into a fully-fledged craze. Famous composers like Carlos Gardel and Roberto Firpo had begun creating tango arrangements that further legitimized it as a respectable dance.
The tone of the music, however, became heavily melancholic during the 1920's and 30's, and ballroom tango began to wane throughout this period as new dance crazes like the samba and foxtrot rose to prominence. The Great Depression, particularly in Argentina, brought restrictions and regulations that further diminished the art form.
However, a wave of populist pride ushered in under Juan Peron's government during the 1940s brought a new golden age of tango that lasted for a decade and saw a variety of tango styles emerge based on geographic factors. Central Buenos Aires developed a style characterized by jagged, seemingly random changes in direction, while the north's style featured movements in long straight lines followed by sudden complex moves. The south's movements were characterized by curves and arcs, with very few straight lines.
Tango's "dark ages" came in 1955 with the fall of Peron's government and the new militaristic regime that replaced it. Upper-class denizens that now controlled the culture did not understand nor embrace the tango and felt it was the byproduct of a dangerous underclass that had embraced Peron's nationalist policies. Part of Peron's downfall was the eradication of anything similarly nationalistic, like tango. Moreover, many of the most prominent tango artists of the time had supported Peron and were thus imprisoned, blacklisted or banished.
When Argentina's military junta fell in 1983, a resurgence of national pride brought about a tango renaissance of sorts. But there were few teachers left to instruct proper tango dancing because suppression of the art (and artists) had lasted nearly three decades, so a more modern form of tango eventually emerged by newer, younger dancers who were largely self-taught or who had to improvise portions of the dance that were unknown.
The renaissance eventually spawned "neotango," a form of music and dance that is distinctly global and sources musical tracks from across the world. Dominant in the twenty-first century, it eschews the strict rules of milonga and flamenco while embracing fusion dances and improvisation. It continues to evolve today.