What are the differences between modern and postmodern music?

In any art
form, one style is almost always a reaction, often an opposing reaction, to the characteristics
of the previous style.  Modern music, which came about at the end of the nineteenth century and
matured in the early twentieth, was an attempt to go beyond the tonal system of music.  Ushered
in by Debussy and taken to its zenith by Stravinsky, modern music brought in music from
non-Western cultures and, at times, sought to completely erase the tonal system.  Some of the
most profound modern music was created while the Western world was being destroyed by World War
I.  A great deal of the music from this period reflects the emotional scars caused by that war. 
Postmodern music, like all other styles before it, sought to correct what it saw as an injustice
done to the art form.  Postmodern music inhabits the time after World War I and continues
through World War II into the late 1950s and early 1960s.  Although it saw a return to the tonal
system, postmodern music retained...