Thus began a long series of jobs including farming, factory work and the lumber business, in the towns of Colemanville, Gap, and Bird in Hand, through which he became self-sufficient at an early age.

At age 20, Welsh headed west to find work as an itinerant carpenter in Cincinnati, Ohio, and Fort Smith, Arkansas.

In January 1848, as soon as his wound had closed, he returned to service, accepting a commission as a second lieutenant in the 11th U.S. Infantry Regiment serving under General Winfield Scott in the Vera Cruz campaign.

In October 1861, he was appointed to command the 45th Pennsylvania Infantry, a three-year regiment, which he had recruited from Center, Tioga, Lancaster, and Mifflin Counties.

In July, the 45th was called north to become part of the IX Corps under Gen. Ambrose Burnside, and Welsh directed the successful rear-guard action during the Union evacuation of Acquia Creek, near Fredericksburg.

When Confederate General Robert E. Lee launched his invasion of Maryland in September 1862, McClellan's Army of the Potomac (of which IX Corps was part) was sent in chase.

On September 17, 1862, at the Battle of Antietam, after first being held in reserved (on account of its losses at South Mountain), Welsh's brigade was placed into action in the afternoon after Burnside exhausted his other troops capturing the bridge that now bears his name.

Against steady opposition, Welsh's troops advanced a mile, entering the village of Sharpsburg (threatening to cut off the Confederate route of escape across the Potomac) before being called back because they could not be supported.

Welsh's performance drew praises from his superiors, and he was promoted to brigadier general of volunteers on November 29, 1862 (confirmed by the Senate on March 13, 1863).