While the Vals followed an independent path by "joining" the Tango with its appearance, the Milonga did not "join" the Tango, but rather "gave birth" to it ... There is little argument today about the Milonga being a fundamental part of the Tango ancestry. It's the mother, indeed.
The bandoneon is the key to the tango sound. Named after its inventor, Heinrich Band (1821–1860), the bandoneon is a large, rather complicated concertina originally developed in Germany for churches that could not afford organs. Around 1870, German and Italian emigrants and sailors brought the instrument to Argentina, where it was adopted into the early genre of tango music.
Tango music can be traced back to the 1850s, but during the Guardia Nueva period, (1925-1935) Tango dancing had started to decline. The musicians began focusing less on the beat and more on the melodies as people were buying records and listening to tango on the radio. Dancing was no longer the primary concern.