which kingdom did he belong to?
England
Thomas Erpingham
Sir Thomas Erpingham ( – 27 June 1428) was an English knight who became famous as the commander of King Henry V's longbow wielding archers at the Battle of Agincourt. 
when did thomas erpingham start his military career? 1368 what was his rank then? He was an esquire and was knighted later.
wiki:14157714
Thomas Erpingham [SEP] Introduction
Sir Thomas Erpingham ( – 27 June 1428) was an English knight who became famous as the commander of King Henry V's longbow wielding archers at the Battle of Agincourt. He was immortalised as a character in the play "Henry V" by William Shakespeare. It is, however, his lengthy and loyal service to John of Gaunt, Henry IV and Henry V, which contributed significantly to the establishment of the House of Lancaster upon the English throne, that is his true legacy.