What is the linking verb in the sentence ? gradpoint answer

Read the excerpt from the dark game: true spy stories from invisible ink to cia moles. yet, by the time robert e. lee surrendered to ulysses grant at appomattox, virginia, some four years later, about 620,000 soldiers had died on the battlefields, more than american battle deaths in all other wars from the revolution through the vietnam war. what is the author’s primary purpose for including this detail? to inform readers about the differences between three military conflicts to persuade readers of the seriousness of the american civil war to entertain readers with tales from behind the lines of battle to encourage readers to visit battlegrounds in appomattox, virginia