How did Robert E. Lee help Abraham Lincoln?

Answer 1

Overall, he didn't help it. He would have helped it greatly if he'd accepted Lincoln's offer of supreme command of the Union army at the beginning. The war would have been over in weeks.

You could say he helped the nation at the end, when he used his authority to stop some of his officers taking to the hills and conducting guerrilla warfare, which might have plagued the South for many years.

Answer 2

He was a valiant and competent commander of the Confederate Army, and helped the southern states by peacefully surrendering. If instead he had retreated to guerrilla warfare tactics, he would have prolonged the war and ensured the even greater destruction of the South.