In Let the Great World Spin, what does "Remember 'nam, Sam" mean, or what does it refer to?This slogan occurs in the prologue of Let the Great...

Whenever you come across the term "Nam" in American
English, it refers to Vietnam.  So this is just an anti-war
slogan.

As you say, this is one of the things people think
the man might be doing.  He might be putting up these protest slogans.  All of the
slogans are opposed to the Vietnam War.

The word "Sam" can
be interpreted in two ways here.  First, you can identify it with "Uncle Sam" -- another
name for the United States.  Or it might just be used to rhyme with "Nam."  In other
words, it would be used to make the slogan more catchy -- like "Get back, Jack" or "even
Steven" -- both of these are cases where the name is only included to make a
rhyme.

Whichever way you want to read the word "Sam" this
is an anti-war slogan because 'Nam refers to Vietnam.