Die Grenzwacht hielt im Osten, о балтийском фрайкоре
The song is from the poem "die Baltenfahne" from Baron Manteuffel-Katzdangen. He was a baltic-german nobleman. It tells about the fight of the German Freikorps, especially the so called "Baltikumer" after WW1. They fought in Latvia and Lithuania against the bolsheviki to defend the German borderlands. Some of the soldiers were of baltic german origin.