Upvote:4
Okay, this cape is, indeed, a US Army cape (note, cape, USN and USMC have similar items of uniform, but are called cloaks).
Officers buy their own uniforms and among the high end items are mess dress (in blue or white) and dress blues. Mess Dress and Evening Mess Dress are the army equivalents to civilian white tie and tails. Due to the cost of mess dress uniforms, when the occasion calls for mess dress, the dress blue or white uniform may be worn with a bow tie as vice the straight tie. The army cape may be worn with either mess dress or dress blues as the occasion warrants, usually a formal affair after regular working hours on or off post.
Note this description and following line drawing from pages 125 & 126 of the 2021 uniform regulation, AR 670-1 below.
The cape has been an article of uniform with little changes, with the exception of specified fabrics as time progressed, for 150 years.