In July 1864, in the third Southern invasion, Confederate troops led by Lieutenant General Jubal Early occupied Frederick and extorted $200,000 ($ in 2019 dollars) from citizens for not razing the city on their way to Washington D.C. Union troops under Major General Lew Wallace fought a successful delaying action, in what became the last significant Confederate advance at the Battle of Monocacy, also known as the Battle that saved Washington. The Monocacy National Battlefield lies just southeast of the city limits, along the Monocacy River at the B. & O. Railroad junction where two bridges cross the stream - an iron-truss bridge for the railroad and a covered wooden bridge for the Frederick-Urbana-Georgetown Pike, which was the site of the main battle of July 1864. Some skirmishing occurred further northeast of town at the stone-arched Jug Bridge where the National Road crossed the Monocacy; and an artillery bombardment occurred along the National Road west of town near Red Man's Hill and Prospect Hall mansion as the Union troops retreated eastward. Antietam National Battlefield and South Mountain State Battlefield Park which commemorates the 1862 battles are located 23 miles and 35 miles respectively to the west-northwest. While Gettysburg National Battlefield of 1863 lies approximately to the north-northeast.