Complex sentence: Infantry is the branch of an army that fights on foot--soldiers specifically trained to engage, fight, and defeat the enemy in face-to-face combat; and thus bear the brunt of warfare, and suffer the greatest number of casualties.
Split into simpler sentences: Infantry is the branch of an army that fights on foot--soldiers specifically trained to engage, fight, and defeat the enemy in face-to-face combat. Infantrymen thus bear the brunt of warfare, and suffer the greatest number of casualties.