In the letter to his son why does robert E. Lee write his letter to his son

Lee wrote it to his son, but it was intended for a wider audience. The internal conflict shown in this letter is that Lee does not want war or secession but refuses to take like up arms against his own state (Virginia).

To explain his ultimate loyalty to Virginia

Explanation:

Robert Edward Lee was the confederate army general during the secession war. Despite being an extremely dedicated commander, Lee was extremely faithful to his home state of Virginia. For this reason, he wrote a letter to his son explaining his decision to leave Virginia and revealing his fears and concerns about secession.