The Army keeps track of what campaigns its units have participated in and what unit awards have been given out.  An excellent reference website is:

http://www.history.army.mil/html/forcestruc/lh.html#terms

For example, to pick a unit at random---

The Headquarters and HQ Battery 32nd Army Air and Missile Defense Command is entitled to:

CAMPAIGN PARTICIPATION CREDIT
World War I
St. Mihiel
Meuse-Argonne

World War II
New Guinea
Leyte

War on Terrorism
Global War on Terrorism

DECORATIONS
Meritorious Unit Commendation (Army), Streamer embroidered SOUTHWEST ASIA 2002-2003
Army Superior Unit Award, Streamer embroidered 1990-1992
Army Superior Unit Award, Streamer embroidered 1993-1994
Philippine Presidential Unit Citation, Streamer embroidered 17 OCTOBER 1944 TO 4 JULY 1945

http://www.history.army.mil/html/forcestruc/lineages/branches/ada/0032aamdcom.htm



Civil War history can get a bit muddled for some of the National Guard Units in some southern states, since the Army decided that campaign credit would be given for units who served in the Confederate Army.  

For example the 149th Infantry of Kentucky has campaign credit for fighting both on the Union and on the Confederate sides since part of the unit remained with the Union and part of the unit fought for the Confederacy, sometimes in the same battles:

CAMPAIGN PARTICIPATION CREDIT
Mexican War
Buena Vista

Civil War (Union service)
Shiloh
Murfreesborough
Chickamauga
West Virginia 1861
Mississippi 1862
Kentucky 1862
Kentucky 1864
Tennessee 1863

Civil War (Confederate service)
Henry and Donelson
Shiloh
Murfreesborough
Chickamauga
Atlanta
Kentucky 1862
Kentucky 1864
Louisiana 1862
Mississippi 1862
Tennessee 1862
Tennessee 1863
Indiana 1863
South Carolina 1865

World War I
Streamer without inscription

World War II
New Guinea
Leyte
Luzon

War on Terrorism
Campaigns to be determined

Company C (Ravenna), 1st Battalion, additionally entitled to:
War on Terrorism
Campaigns to be determined

Company D (Middleboro), 1st Battalion, additionally entitled to:
War on Terrorism
Campaigns to be determined

DECORATIONS
Philippine Presidential Unit Citation, Streamer embroidered 17 OCTOBER 1944 TO 4 JULY 1945
Company C (Ravenna), 1st Battalion, additionally entitled to:
Army Superior Unit Award, Streamer embroidered 2003

http://www.history.army.mil/html/forcestruc/lineages/branches/inf/0149in.htm

Fight as Kentuckians!

E Co., 1/149th (Sommerset, KY) should have both Iraq ands Afganistan streamers.

THE LIST Have been PCed to death

I met a guy in 04 who was an 82nd ABN vet (like me) we talked for w while he had 2 combat tours one in VITNAM IN 66 ONE DETROIT IN 63

THERE IS NO NOTE OF A DEPLOYMENT TO DETROIT IN 82ND RECORDS BUT I CHECKED DETROIT NEWS PAPERS- AND THE DETROIT POLICE CHIEF IN 63 WAS A FAMILY FRIEND BOTH CONFIRMED THE 82ND WAS THERE.  SO EXPECT WHITE Washing.

My old unit isn't even listed there.