After a few months at the cavalry barracks in Carlisle, Pennsylvania, Kearny was assigned to the staff of General Winfield Scott, soon becoming his aide-de-camp.

Kearny, disappointed with the lack of fighting he was seeing in the Army, resigned his commission in 1846, but returned to duty a month later at the outbreak of the Mexican–American War.

The unit was originally stationed at the Rio Grande but soon became the personal bodyguard for General Scott, the commander-in-chief of the Army in Mexico.

After the failure of his marriage, frustrated with the slow promotion process of the Army, Kearny resigned his commission in October of that year.

In Paris, Kearny fell in love with a New York City woman named Agnes Maxwell, but was unable to marry her because his first wife would not grant him a divorce.

When the American Civil War broke out in 1861, Kearny returned to the United States and was appointed a brigadier general, commanding the First New Jersey Brigade, which he trained.

The Army had been reluctant to restore his commission due to his disability, but the shocking Union defeat at the First Battle of Bull Run made them realize the importance of seasoned combat officers.

At Williamsburg, as he led his troops onto the field, Kearny shouted (in a notable quote), "I'm a one-armed Jersey son-of-a-gun, follow me!"

And in full view of all responsible for such declaration, I say to you all, such an order can only be prompted by cowardice or treason.Kearny is credited with devising the first unit insignia patches used in the U.S. Army.

In the summer of 1862, he issued an order that his officers should wear a patch of red cloth on the front of their caps to identify themselves as members of his unit.

Members of other units picked up on the idea, devising their own insignia, and these evolved over the years into the modern shoulder patch.

The Union army retreated toward Washington and fought with the pursuing Confederate corps under Stonewall Jackson on September 1, 1862, at the Battle of Chantilly.

Encountering Confederate troops, Kearny ignored a demand to surrender and, while he tried to escape on horseback, a "half dozen muskets fired" and he was shot with a Minié ball that entered his hip and came out his shoulder, killing him instantly.

Hill, upon hearing the gunfire, ran up to the body of the illustrious soldier with a lantern and exclaimed, "You've killed Phil Kearny!

Confederate soldiers quickly proceeded to strip Kearny's body of his coat, boots, pocket watch, papers, and other items of value.

However, after it was realized who the deceased was, Robert E. Lee ordered all of his belongings returned over the objections of poorly clad soldiers who protested that a dead man no longer needed a warm coat and boots.

The re-interment drive was spearheaded by Medal of Honor recipient Charles F. Hopkins, who had served under General Kearny in the First New Jersey Brigade.