From West Point he was posted to the 1st Artillery, very quickly becoming a second lieutenant. He spend the first years of his career on the Canadian border, then an area of much tension. In 1841 he returned to West Point as a tactical officer, where he remained until the start of the Mexican War. While at West Point he was promoted to first lieutenant.
He started the war as aide-de-camp to General Wool (October 6 1845). In that capacity he distinguished himself at the battle of Buena Vista (22-23 February 1847), and was promoted to brevet captain. After the Mexican War, he served in a variety of staff roles, including a period at Army headquarters, rising to the rank of major in 1856.
Inevitably this service at headquarters meant that he was well known to General Winfield Scott, the head of the army at the outbreak of the civil war. On 14 May 1861 he was promoted to brigadier-general. At first he served under General Mansfield, in the army building around Washington. When part of that army was sent across the Potomac to the Virginia bank (23-24 May 1861), McDowell was appointed to command it, as head of the new Department of Northeastern Virginia (27 May). This army would become more famous as the Army of the Potomac.
McDowell was under intense pressure to act. There was a Confederate army at Manassas Junction, only twenty miles from Washington. The majority of the men in his army had enrolled for three months, and their time in the army would soon be coming to an end. General Scott’s preferred plan was to squeeze the Confederacy to death by blockading her ports and clearing the Mississippi (the ‘anaconda plan’. His plan was very close to what eventually happened, but the time was not yet right for such long-term planning. President Lincoln came under a great deal of pressure to act immediately against the army at Manassas Junction.
Lincoln decided that it was worth taking the risk. Other Confederate armies had not fought well. A large force at Harper’s Ferry, under General Joseph Johnston, had retreated at the first sight of a Federal column. If McDowell’s men succeeded, then they might have gone a long way towards winning the war. If they failed, then hundreds of thousands of men enlisted for three years were already filling the ranks of a new army that would take their place.
If anything McDowell’s extensive experience and military training now became a weakness. His plan for the upcoming campaign needed experienced troops. He had raw recruits, most only signed up for three months. Admittedly, so did the southern commander at Manassas, General Beauregard, but their defensive task was rather more straightforward. McDowell was well aware of the limited experience and training of his men, and would have preferred to wait until the new three-year men could be trained.
McDowell’s plan was for his army to advance towards the Bull Run area from Washington, while General Patterson at Harper’s Ferry made sure that the Confederate Army of the Shenandoah under Joseph Johnston was not able to move east to reinforce Beauregard. Unfortunately, Patterson failed to achieve this, and by the time McDowell launched his attack, most of Johnston’s army was already with Beauregard, while the final brigade arrived on the day of the battle. That attack was launched on 21 July 1861 (First Bull Run or Manassas), and came very close to success, but McDowell’s inexperienced army could not quite take the opportunities it was given. Finally, Beauregard launched a ferocious counterattack, and most of McDowell’s army simply dissolved.
Ironically, one of McDowell’s failures during the battle now helped him. He had been unable to get his reserve brigades into the action during the day. Now they formed a defensive line at Centreville, and waited for the Confederate counterattack. No such attack came. The Confederate army was in little better shape than McDowell’s. Its commanders were well aware of the strength of the defences around Washington.
McDowell was quickly replaced by George B. McClellan. His conduct of the battle had been sufficiently competent for him to be retained as a divisional commander in McClellan’s Army of the Potomac. However, they were never to fight together. In the preparations of McClellan’s Peninsula campaign, McDowell was promoted to major-general of volunteers and give command of the I Corps. In the initial plans this corps was to move first, but after the Confederate retreat from Manassas Junction altered McClellan’s plans, the order of movement was changed.
This meant that it was McDowell’s corps that was retained at Washington when Lincoln decided that the measures McClellan had taken to defend the capitol were inadequate. McDowell himself protested against this decision, but that did not prevent McClellan from numbering McDowell amongst his enemies in Washington. Matters were not helped when McDowell was given command of a new Department of the Rappahannock. Although large parts of McDowell’s corps joined McClellan, a sizable part of the Union army was effectively removed from the crucial battles on the Peninsula. When Stonewall Jackson began to win his victories in the Shenandoah Valley, McDowell’s corps was once again diverted from aiding McClellan, once again against McDowell’s advice.
Just as McClellan’s campaign was reaching its conclusion during the Seven Days’s Battles, a new army was being formed in northern Virginia. The three separate armies under McDowell, Fremont and Banks were combined to for a new Army of Virginia under General John Pope. While Fremont resigned, McDowell was happy to serve under Pope, and soon became close to his new commander.
Ironically, Pope’s campaign also ended in defeat at Manassas. At the Second Battle of Bull Run (29-30 August 1862), McDowell had command of Pope’s biggest corps, the Third. This corps played a very minor role in the fighting on 29 August. On that day Pope was attacking Stonewall Jackson’s wing of the Confederate army, unaware that Longstreet and Lee were approaching his left wing. McDowell’s men were only really in place at the end of the day, too late to make any significant contribution to the attack on Jackson’s positions. The next day his two divisions were scattered around the battlefield, ready to take part in what Pope expected to be the final push against Jackson, and so McDowell was given little chance to redeem himself, although he had sent Pope a message warning him of Longstreet’s impending attack. In Pope’s plans for 30 August, McDowell had actually been assigned to command the expected pursuit of Jackson’s broken forces!
Pope’s determined attack early on 30 August came close enough to success for Jackson to call for reinforcements. Longstreet and Lee’s response was to launch their counterattack on Pope’s weakened left wing. McDowell’s planned role would not been needed. Instead, he and Pope found themselves managing another retreat from Bull Run. This time things were different. This was the three-year army that McDowell had wanted to wait for in the previous year. It fell back but did not rout. Pope was able to keep most of his army together, even it was badly demoralised by now. McDowell was given command of the rearguard, but no pursuit was mounted. Pope was able to hand his army almost intact to General McClellan, who was restored to command for the campaign that saw Lee’s first invasion of the north defeated at Antietam.
Second Bull Run inevitably ended McDowell’s active military career. His movements on the first day of the battle had been sufficiently poorly handled to even raise doubts about his loyalty. In the aftermath of the battle, he was relieved of his command. He was later cleared by an inquiry, and remained in active service in the army, but not in the field. After being cleared by the inquiry, he was appointed to command the Department of the Pacific (1864). In that role he was based at San Francisco, where he later retired. After the war he remained in the army, eventually reaching the rank of major-general in the regular army in 1872.
McDowell was probably unfortunate in the roles that fell to him. His first ever independent command was the Army of the Potomac! His only major failing before First Bull Run was the slowness of his movements, hardly a unique failing amongst Union commanders at that time. The disaster at Second Bull Run was largely due to Pope’s misreading of the situation. McDowell was simply one of many commanders to be promoted above their capacity early in the war.