For more than two decades, Andrew Carroll has been urging people not to throw away the letters written by family members or friends during a time of war. Carroll, director of the Center for American War Letters at Chapman University in California, considers them an irreplaceable link to tie families together and open a door of understanding. “Younger generations are reading these letters,” Carroll says, “and asking questions and saying, ‘Now I understand what you endured, what you sacrificed.’ ”

When the apostle Paul was imprisoned in Rome and knew his life would soon end, he wrote a letter to a…