The department of war publishes the 1940 edition of of the Army Field Manual Paragraph 43, under the title “Rules of Land Warfare,” says,

       National flags, insignias and uniforms as a ruse [false flags]- in practice it has been authorized to make use of these as a ruse.  The foregoing rule (Article 23 of the Annex of the IVth Hague Convention), does not prohibit such use, but does prohibit their improper use.  It is certainly forbidden to make use of them during a combat.  Before opening fire upon the enemy, they must be discarded

       [added 7/24/2022] Thanks to Jim Lorenz and Bill Holmes for this entry.