The essay begins by questioning whether or not England should build a statue to Oliver Cromwell. The purpose of the essay is really to discuss whether or not the US should build a statue to Robert E. Lee. (Please keep in mind that Mr Adams fought on the Union side against Lee).
Adams' answer is unequivocally "yes."
He goes through a long argument about how Lee was not a traitor. For if we wish to call Lee a traitor, we would have to call Washington, Cromwell, William of Orange and Hampden traitors as well. Lee was loyal to his state, which was where he believed his primary loyalty lay.
Then Adams tries to make a distinction between Virginia's decision to secede and other Cotton States' decisions to secede. The latter states seceded when Lincoln won the election. Virginia did not. Virginia believed in secession (as did everyone who ratified the Constitution, according to Mr Adams). Virginia was willing to let the other states peacefully secede, but did not wish to secede with them. Only after the US government tried to re-supply Sumter, an act of war against a sovereign state (i.e. South Carolina), according to the logic of Virginia and the original understanding of the Constitution, did Virginia rebel. According to Virginia, the North had effectively changed the Constitution at that point and Virginia seceded to defend the original Constitution. Mr Adams understands this argument but sees it as hopeless outdated and out-of-touch. Nevertheless, he sees it as consistent. Lee then went with his state.
Adams then goes into a discussion of how fortunate the US was to have Grant and Lee as the men who first brokered peace and later stood by to ensure that the peace was enforced. This is a topic that was dealt with very nicely in book-length here. He also spends time praising Lee's character.
Adams goes on to predict that Lee will have a statue in his old home, Arlington Cemetery. It's interesting that Mr Adams has so far, been wrong.