Maj. Gen. John C. Frémont’s and his 11,000-man Mountain Department army were tasked with keeping Maj. Gen. Thomas J. "Stonewall" Jackson's two-division force engaged in the Shenandoah Valley and unable to join with Robert E. Lee's army defending Richmond during the Peninsula Campaign against the Confederate capital. In early June, Jackson's men under Maj. Gen. Richard S. Ewell were encamped in the vicinity of Cross Keys on the South Fork of the Shenandoah River. Union screening cavalry approached Jackson from Harrisonburg early on June 8th and made contact with the Confederate defenders. Deploying his infantry east and west of the Port Republic Road, Frémont moved south toward Jackson. Brig. Gen. Julius H. Stahel's brigade on the Union left collided with Brig. Gen. Isaac Trimble's brigade. Stahel was driven back in confusion by a surprise volley from Trimble's men and ordered up Union artillery which engaged Confederate reinforcements. On the Union right, two more brigades attacked Confederate defenders along Mill Creek. After feeling out other parts of the Confederate line, Frémont withdrew to the Keezletown Road under protection of his batteries. The next day, Trimble’s brigade held Frémont at bay, while the rest of Ewell’s force crossed the river to assist in the defeat of Union forces at nearby Port Republic. Rebel victories at Cross Keys and Port Republic weakened Frémont and enabled Jackson to support Lee outside of Richmond.