Why did robert e. lee lead the confederate army into maryland and the battle of antietam?

The answer is he wanted to inspire a pro-confederate uprising

Robert E. Lee led the Confederate army to Maryland after shuffling all the possibilities he had because it was the last real option, and his plans were approved by President Davis. And although before the military strategy of the Confederation had been strictly to defend the borders and the territory of the Confederation, it was a good time for a change, in which there were great risks, but also glorious possibilities.  

Robert E. Lee led the Confederate army to Maryland after shuffling all the possibilities he had because it was the last real option, and his plans were approved by President Davis. And although before the military strategy of the Confederation had been strictly to defend the borders and the territory of the Confederation, it was a good time for a change, in which there were great risks, but also glorious possibilities.  

In September of 1862, Lee actually wanted to try and to inspire a pro-confederate uprising, and he launched what would be a surprise invasion in Maryland. But when a misplaced dispatch describing the details of the plan fell into the hands of the Union, what resulted was the bloodiest day of the war at the Battle of Antietam. Technically, the two armies fought to a draw, but Lee retreated under cover of darkness to Virginia.