Complex sentence: Henry arrives at the fenced-in camp only to realize that Col. Curtis plans to kill all of those quarantined, so (Henry) convinces Owen to prevent this by inviting the larger U.S. Army, which arrives, relieves Curtis and takes over the entire operation.
Split into simpler sentences: Henry arrives at the fenced-in camp only to realize that Col. Curtis plans to kill all of those quarantined. Underhill to prevent this by inviting a larger U.S. Army unit to relieve Col. Curtis and takes over the entire operation.