John D. Sebastian was born 1842 in Phillips County, Arkansas, son of Joseph W. and Sarah (Connally) Sebastian. The family appears in the 1850 Census for Big Creek TWP, Phillips County, and the 1860 Census for Hickory Ridge TWP, Monroe County. There were only two Sebastian families residing in Monroe County in 1860, that of Joseph W. and his son William H. John D. was in his father’s household. You noted that Clarendon was mentioned in the soldier’s letter of 14 Jul 1862 – Clarendon is in Monroe County. John D. Sebastian first enlisted 28 May 1862 in Company A (Harris Guards – Monroe County), Josey’s 15th Arkansas Infantry. He is on record as enlisting again on 17 Jun 1862 in Company E, Cocke’s Arkansas Infantry, at Clarendon, Monroe County, enrolled by W. B. Meeks for the war. He next appears as a private in the Texas Spy Company. This was a roster of soldiers captured at Arkansas Post on 11 Jan 1863. I assume the mention of the Sebastian family living near Clarendon gives credence that this John D. and the others are one and the same. I did not find any record revealing how he got to the Texas unit – you probably already know how he got there, or perhaps someone else on this forum can help with that. I found no reference to a Tas Sebastian.

Good luck in your search.

Sources:

CSR, NARA

Cassandra Wade, “Wade Family 1776 to Present,” Ancestry.com; J.W. Sebastian Household, 1850 U.S. Census, Big Creek TWP, Phillips County, Arkansas Population Schedule, Page 149A, Lines 9-17, Dwelling 99, Family 99, National Archives Micropublication M432, Roll 29, Ancestry.com Image 13; Jos. W. Sebastian Household, 1860 U.S. Census, Hickory Ridge, Monroe County, Arkansas Population Schedule, Page 837, Lines 25-33, Dwelling 517, Family 517, National Archives Micropublication M653, Roll 46, Ancestry.com Image 7