Ancestry does not list a a card filed for Robert H Martin in Erath County TX. There is no Robert H Martin with a card in all of Texas. There is a Robert Martin b. Oct 1, 1876 filed in San Antonio, Bexar County TX.Filed Sept 12, 1918.
There is a 3 Vol set of a Books titled "Soldiers of the Great War", Ancestry has it as a database titled "American Soldiers of WWI". It claims to list all men that were KIA, WIA, DOD, DOA, DOW in WWI. Listed by state and rank and some jobs like Buglers, Wagoners, then the city they were lived before war. For all of Texas there is only one surname Martin with first name Robert, middle initial A. of Rogers, (Bell County) TX listed as a Private under KIA.
On another message board you have the cemetery (in Somervell Co.)a Robert H. Martin that was born Oct, 8 1876 but died Feb 26, 1918. His father died in 1917 so that makes me believe they both died in the Flu Epidemic/Pandemic that was occurring at the same time as the war. Since RH headstone lists parents and not a spouse I would also believe he was single at time of death and probably not in the military.