The conflict had faded from popular memory within fifteen years of the end of the U.S - Mexico war, but it has not disappeared, due to the outbreak of the U.S. Civil War.
A. The conflict
had faded from popular memory within fifteen years of the end of the U.S - Mexico war, but it has not disappeared, due to the outbreak of the U.S. Civil War X
-past perfect tense not required...there's no need to demarcate the when the US-Mexico war occurred relative to the Civil War b/c we get the 'within fifteen years'
B. The conflict has faded from popular memory but it has not disappeared,
due to the outbreak of the U.S. Civil War within fifteen years of end of the U.S - Mexico war X
-meaning change...
C. Within fifteen years of the U.S - Mexico war’s end, the conflict faded from popular memory, but it did not disappear, due to the outbreak of the U.S. Civil War
CORRECTD. Within fifteen years of the U.S - Mexico war’s end, the conflict
had faded from popular memory, but it
did had not disappeared, due to the outbreak of the U.S. Civil War X
E. The conflict has not disappeared but it has faded from popular memory due to the outbreak of the U.S. Civil War within fifteen years of end of the U.S - Mexico war X