Complex sentence: In the 20th century, the Philadelphia Experiment opened a gateway into another dimension, first freeing the aliens Horse refers to as "the Legion", and an independent government group spent the subsequent decades in contact with the aliens following the Roswell UFO incident.
Split into simpler sentences: In the 20th century, the Philadelphia Experiment opened a gateway into another dimension, first freeing the aliens Horse refers to as "the Legion". An independent government group spent the subsequent decades in contact with the aliens following the Roswell UFO incident.