Complex sentence: By the late 19th century, the concept of fighting in formation was on the wane, and the distinctions between skirmishers and heavy infantry began to disappear, although airborne forces still fulfill the need for a highly mobile screening force.
Split into simpler sentences: By the late 19th century the concept of fighting in formation was on the wane and the distinctions between skirmishers and heavy infantry began to disappear. Although airborne forces still fulfill the need for a highly mobile screening force.