Two battles proved to be the undoing of Lee's attempt to protect Richmond in the latter days of the Civil War?

Answer 1

I would say Fort Stedman (March 25 1865) and Five Forks (April 1, 1865). Fort Stedman was Lee's last attack, on a strong point in the Yankee lines opposite his own. It was a surprise attack in the night, and hand-picked Confederate troops were able to chop their way through the abatis (sharpened wooden stakes, used something like barbed wire) and overrun the Fort in a rush. Then daylight came, and neighboring Yankee strong points were able to bring the Rebels in the Fort under fire. Only a few Rebels were able to get away back to their own lines, and the rest had to surrender. This was the end of any hope Lee might have entertained of being able to force the Yankees to pull back and lift the siege. President Lincoln was with his Army that day, visiting General Grant at City Point, and went to have a look at the several thousand Rebel prisoners in their stockade. So the Rebels got a look at him, too, three weeks before he was murdered.

Beginning in the summer of 1864, the Siege of Richmond and Petersburg was like a dress rehearsal for WWI - there were trench lines stretching from north of Richmond to south of Petersburg - more than fifty miles. Grant had vastly more men than Lee, though, and kept extending his lines to his left (the Confederate right) below Petersburg. This forced Lee to also extend his own lines, which meant stretching the thin Rebels out farther and farther every time Grant did this. The Confederates were completely dependent on two railroad lines coming into Petersburg from the south for their supplies, and if they lost control of these railroads they could not continue to fight there where they were. Each extension of Grant's lines southward brought the Yankees closer to breaking these vital Rebel supply lines, and eventually at Five Forks, they did. The Rebel counterattacks were unsuccessful, and so, the next day, Lee's Army pulled out, after ten months of resisting the siege, and tried to get away to the west.