The United States Guards were a formation of the National Army created to guard strategic installations and areas in order to free infantry regiments for war service in the First World War.

The Guards were created on 22 December 1917 and were controlled by the Chief of the Militia Bureau. They eventually comprised 48 battalions of men who were either too old or physically unfit for active service.

The Guards were disbanded following the end of the war in 1920. The experience led the US Army to not wish to bear the financial burden of paying for troops whose only use was internal security. The idea of reforming the US Guards was discussed in 1941 with the National Guard Bureau replying that the War Department

United States Department of War

The United States Department of War, also called the War Department , was the United States Cabinet department originally responsible for the operation and maintenance of the United States Army...

 did not want Federal troops for internal security duties; this function would be performed by State Guards.

American Civil War

During the War Between the States a Colonel H.P. Montgomery offered to recruit a regiment in New York with the title of "United States Guards" but his offer was not accepted.

Reference

National Guard Bureau Military Protection, United States Guards: The Use of Organized Bodies in the Protection and Defense of Property During Riots, Strikes, and Civil Disturbances 1919
United States.