As Mr. Gallagher mentioned, Mr. Freeman relates that Lee's decision was by no means an easy one.
When, in March, 1861, he returned to Washington, it appeared that Virginia would not secede. A Constitutional amendment, preserving slavery in the states where it currently existed, had been approved by the House of Representatives on February 28, 1861 and had just received the necessary two-thirds vote in the Senate.
<Douglas S. Freeman, R. E. Lee, a Biography. (New York: Charles Scribner & Sons,. 1934), Vol. I, p. 433.>
Even as late as April 3 it appeared that a clash would be avoided over Fort Sumter. On April 4 a test vote in the Virginia convention showed a majority of two-to‑one against secession. <Freeman, Vol. I, p. 434>
It was only when he met with Francis P. Blair on April 18 that Lee's course became clear. Blair told him that Lincoln was going to enforce the law with a large army and that he was authorized by Lincoln to offer Lee command of that army. Lee described his reaction, "I declined the offer he made me to take command of the army that was to be brought into the field, stating as candidly and as courteously as I could, that though opposed to secession and deprecating war, I could take no part in an invasion of the Southern States."<Freeman, Vol. I, p. 437>