Complex sentence: By World War I, drum kits were characterized as a very large bold marching military bass drums and many percussion items suspended on and around them, and they became a central part of jazz music, specifically (but not limited to) Dixieland.
Split into simpler sentences: By World War I, drum kits were often marching band-style military bass drums with many percussion items suspended on and around them. Drum kits became a central part of jazz music, specifically (but not limited to) Dixieland.