The Grand Army of the Republic (GAR) was a veteran's organization for honorably discharged Union veterans from the American >Civil War. Founded in 1866, the GAR became a powerful special interest group in American society in the late 1800s, with political candidates and organizations vying for a GAR endorsement, and it also served as an important way for veterans to network and maintain connections with each other. The organization was dissolved in 1956, with the death of the last Civil War veteran who fought on the side of the Union.

The American Civil War was a dramatic and traumatic event, throwing together soldiers from very diverse backgrounds and communities. After the cessation of the war, when the United States began to rebuild itself, many of these veterans wished to keep in contact with each other, using their shared experiences as a basis for fellowship. In 1866, Benjamin F. Stephenson established the Grand Army of the Republic in Decatur, Illinois, specifying that membership would be open to all honorably discharged soldiers who had fought on the side of the Union.

By 1890, the GAR had over 400,000 members, organized into regional Posts. Veterans would periodically get together for annual “encampments,” huge parties which often went on for several days, allowing veterans to enjoy each other's company while having a good time eating, drinking, and playing various sports. The GAR also offered service and assistance to veterans in need, and maintained several cemeteries for Union veterans; GAR officials were also a driving force behind the establishment of Memorial Day, which was originally intended to commemorate the dead of the Civil War.

In the late 1800s, an organization known as the Sons of Union Veterans of the Civil War (SUVCW) was founded, to ensure that the legacy of the GAR would be passed on, even after all Union veterans passed away. This organization continues to be active, promoting the maintenance of Civil War history in museums and re-enactments. Women who are descended from Union Civil War veterans can join the SUVCW auxiliary.

The United Confederate Veterans could be considered the equivalent veterans organization for those who fought on the side of the Confederacy. It is probably less well-known because its soldiers fought on the losing side, although it also contributed a variety of services to its members, ranging from burial benefits to annual gatherings.

One interesting legacy of the GAR can be seen in many old American graveyards; the organization customarily marked the graves of Union veterans with a distinctive star which is known as a GAR Star. The tradition of marking graves with stars is also kept up by the Veterans of Foreign Wars, which puts up its own VFW stars on the graves it maintains.