He was a supporter of eventual President Abraham Lincoln, and was considered for a cabinet position in the Lincoln administration.
 However, after the Civil War broke out, Curtis was appointed colonel of the 2nd Iowa Infantry on June 1, 1861, prompting him to resign his congressional seat on August 4 of that year.
He was subsequently promoted to brigadier general, with the promotion backdated to May 17, 1861.

After organizing the chaos in St. Louis, Missouri, Curtis was given command of the Army of the Southwest on December 25, 1861, by Maj.
Gen. Henry W. Halleck.
The Army originally consisted of three divisions, the 1st commanded by Brig.
Gen. Franz Sigel, the 2nd by Brig.
Gen. Alexander Asboth, and the 3rd by Col.
Jefferson C. Davis.
However, Sigel, a native German who held significant influence amongst the many German immigrants in the army, threatened to resign over having not been appointed to command of the army himself.
Curtis subsequently gave him overall command of the first two divisions, consisting mainly of German immigrants, while creating a 4th Division commanded by Col.
Eugene A. Carr.

Curtis moved his headquarters south to Rolla, Missouri, to solidify Union control in Arkansas.
In March 1862, his army won the Battle of Pea Ridge in northwest Arkansas.
His success made him pensive rather than triumphant.
A few days after the battle he wrote, "The scene is silent and sad.
The vulture and the wolf now have the dominion and the dead friends and foes sleep in the same lonely graves."
He was promoted to major general for his success, effective March 21, 1862.
On the same day in late March that he heard about his promotion, he also found out that his twenty-year-old daughter Sadie died of typhoid fever in St. Louis.

After Pea Ridge, Curtis' small army moved east and invaded northeast Arkansas, capturing the city of Helena, Arkansas in July.
In September, Curtis was given command of the District of Missouri, but Lincoln was soon forced to reassign him, after Curtis's abolitionist views led to conflict with the governor of Missouri.
 He was reassigned to command the Department of Kansas & Indian Territory.

In October 1863, his son Major Henry Zarah  Curtis, adjutant to Brig.
Gen. James G. Blunt, was killed by Quantrill's Raiders.
In this surprise attack at the Battle of Baxter Springs, Quantrill's men wore Federal uniforms and gave no quarter.
Samuel Curtis named Fort Zarah in memory of his son.

In 1864, Curtis returned to Missouri, fighting against the Confederate invasion led by Maj.
Gen. Sterling Price.
Curtis gathered the forces of his department together, including several regiments of Kansas State Militia, calling his force the Army of the Border.
 Price's incursion was halted by Curtis' victory at the Battle of Westport.
Curtis was then reassigned to a completely different armed conflict, commanding the Army's "Department of the Northwest," which was in the closing phase of a military response to uprisings in southern Minnesota and Dakota Territory by Native Americans against settlers.
He was a supporter of eventual President Abraham Lincoln, and was considered for a cabinet position in the Lincoln administration.  However, after the Civil War broke out, Curtis was appointed colonel of the 2nd Iowa Infantry on June 1, 1861, prompting him to resign his congressional seat on August 4 of that year. He was subsequently promoted to brigadier general, with the promotion backdated to May 17, 1861.
After organizing the chaos in St. Louis, Missouri, Curtis was given command of the Army of the Southwest on December 25, 1861, by Maj. Gen. Henry W. Halleck. The Army originally consisted of three divisions, the 1st commanded by Brig. Gen. Franz Sigel, the 2nd by Brig. Gen. Alexander Asboth, and the 3rd by Col. Jefferson C. Davis. However, Sigel, a native German who held significant influence amongst the many German immigrants in the army, threatened to resign over having not been appointed to command of the army himself. Curtis subsequently gave him overall command of the first two divisions, consisting mainly of German immigrants, while creating a 4th Division commanded by Col. Eugene A. Carr.
Curtis moved his headquarters south to Rolla, Missouri, to solidify Union control in Arkansas. In March 1862, his army won the Battle of Pea Ridge in northwest Arkansas. His success made him pensive rather than triumphant. A few days after the battle he wrote, "The scene is silent and sad. The vulture and the wolf now have the dominion and the dead friends and foes sleep in the same lonely graves." He was promoted to major general for his success, effective March 21, 1862. On the same day in late March that he heard about his promotion, he also found out that his twenty-year-old daughter Sadie died of typhoid fever in St. Louis.
After Pea Ridge, Curtis' small army moved east and invaded northeast Arkansas, capturing the city of Helena, Arkansas in July. In September, Curtis was given command of the District of Missouri, but Lincoln was soon forced to reassign him, after Curtis's abolitionist views led to conflict with the governor of Missouri.  He was reassigned to command the Department of Kansas & Indian Territory.
In October 1863, his son Major Henry Zarah  Curtis, adjutant to Brig. Gen. James G. Blunt, was killed by Quantrill's Raiders. In this surprise attack at the Battle of Baxter Springs, Quantrill's men wore Federal uniforms and gave no quarter. Samuel Curtis named Fort Zarah in memory of his son.
In 1864, Curtis returned to Missouri, fighting against the Confederate invasion led by Maj. Gen. Sterling Price. Curtis gathered the forces of his department together, including several regiments of Kansas State Militia, calling his force the Army of the Border.  Price's incursion was halted by Curtis' victory at the Battle of Westport. Curtis was then reassigned to a completely different armed conflict, commanding the Army's "Department of the Northwest," which was in the closing phase of a military response to uprisings in southern Minnesota and Dakota Territory by Native Americans against settlers.
Samuel Ryan Curtis