While the vast majority of number of volunteer units during the American Civil War

American Civil War

The American Civil War was a civil war fought in the United States of America. In response to the election of Abraham Lincoln as President of the United States, 11 southern slave states declared their secession from the United States and formed the Confederate States of America ; the other 25...

 were enlisted by the states, a small number were enlisted directly by the Federal government. These units included regiments like Hiram Berdan

Hiram Berdan

Hiram Berdan was an American engineer, inventor and military officer, world-renowned marksman, and guiding force behind and commanding colonel of the famed United States Volunteer Sharpshooter Regiments during the American Civil War...

's U.S. Sharpshooters and "galvanized Confederates" recruited from Confederate

Confederate States of America

The Confederate States of America was a government set up from 1861 to 1865 by 11 Southern slave states of the United States of America that had declared their secession from the U.S...

 prisoners of war recruited to fight Indians

Indigenous peoples of the Americas

The indigenous peoples of the Americas are the pre-Columbian inhabitants of North and South America, their descendants and other ethnic groups who are identified with those peoples. Indigenous peoples are known in Canada as Aboriginal peoples, and in the United States as Native Americans...

 on the frontier

American Old West

The American Old West, or the Wild West, comprises the history, geography, people, lore, and cultural expression of life in the Western United States, most often referring to the latter half of the 19th century, between the American Civil War and the end of the century...

 rather than remain in camps.

Sharpshooters

• 1st United States Volunteer Sharpshooter Regiment
  1st United States Volunteer Sharpshooter Regiment
  The 1st United States Volunteer Sharpshooter Regiment was an infantry regiment that served in the Union Army during the American Civil War.-Service:The 1st U.S...
  
  
• 2nd United States Volunteer Sharpshooter Regiment
  2nd United States Volunteer Sharpshooter Regiment
  The 2nd United States Volunteer Sharpshooter Regiment was an infantry regiment that served in the Union Army during the American Civil War. From 1861 to January 1863 they were members of the "First Iron Brigade" also known as the "Iron Brigade of the East"....
  
  
• Birge's Western Sharpshooters
  66th Illinois Volunteer Infantry Regiment
  The 66th Illinois Veteran Volunteer Infantry Regiment originally known as "Birge's Western Sharpshooters" and later as the "Western Sharpshooters-14th Missouri Volunteers", was a specialized regiment of infantry sharpshooters that served in the Union Army during the American Civil War.-...
  
    (variously known as: "Birge's WSS" from Nov 1861-March 1862; "WSS-14th Missouri Vols" from March 1862- late 1862; and "66th IL Vol Inf (WSS)" from late 1862-July 7 1865. Were initially sworn into service as a non-State aligned unit, as the 1st & 2nd USVSS were, under the personal authority of MG Fremont, Commander of the Dept of the West (placing the regiment in de facto direct U.S. service). Fremont was later found to have exceeded his authority by directly commissioning the Sharpshooters without the authority of the President or a State Governor. In March 1862 they were assigned to the control of the State of Missouri, and the officers were given Missouri commissions. Later the unit was transferred to the control of the state of Illinois)

Infantry

• 1st United States Volunteer Infantry Regiment
• 2nd United States Volunteer Infantry Regiment
• 3rd United States Volunteer Infantry Regiment
• 4th United States Volunteer Infantry Regiment
• 5th United States Volunteer Infantry Regiment
• 6th United States Volunteer Infantry Regiment