Final answer:
The commander of the Union Army in 1861 was General George B. McClellan, who was nicknamed 'Young Napoleon'. His cautious approach to battle was a source of contention with President Lincoln and Congress, leading to a push for more aggressive action.
Step-by-step explanation:
In 1861, the commander of the Northern or Union Army was General George B. McClellan. McClellan took command of the Army of the Potomac, the Union fighting force operating outside of Washington, D.C. His approach to engaging the Confederate forces was one of caution, as he overestimated their strength and was hesitant to risk his troops in battle. This cautiousness earned him the nickname 'Young Napoleon' due to his initial perceived resemblance in strategic planning and potential to the famous French military leader Napoleon Bonaparte. However, this also led to frustration from President Abraham Lincoln and Congress, who were eager for decisive action.
Despite McClellan's reluctance, President Lincoln and Secretary of War Edwin Stanton, driven by the urgent need to address the secession issue, ordered an invasion of the Confederacy with the intent of capturing its capital, Richmond, Virginia. This order marked a new chapter in the Civil War, with the Union escalating their efforts by enlisting a million men for three-year terms to serve in the Potomac Army.