Grant maneuvered again, meeting Lee at the North Anna River (Battle of North Anna, May 23–26). Here, Lee held clever defensive positions that provided an opportunity to defeat portions of Grant's army, but illness prevented Lee from attacking in time to trap Grant. The final major battle of the campaign was waged at Cold Harbor (May 31 – June 12), in which Grant gambled that Lee's army was exhausted and ordered a massive assault against strong defensive positions, resulting in disproportionately heavy Union casualties. Resorting to maneuver a final time, Grant surprised Lee by stealthily crossing the James River, threatening to capture the city of Petersburg, the loss of which would doom the Confederate capital. The resulting Siege of Petersburg (June 1864 – March 1865) led to the eventual surrender of Lee's army in April 1865 and the effective end of the Civil War.