Complex sentence: It's also impossible that Captain Parmenter would have been able to discover through army records that Duffy had been "killed in action" because in 1836, Texas was not yet part of the United States and became an independent nation soon after the battle.
Split into simpler sentences: It's also impossible that Captain Parmenter would have been able to discover through army records that Duffy had been "killed in action". In 1836, Texas was not yet part of the United States, and only became an independent nation after the battle.