Complex sentence: Infantry is the branch of an army who fight on foot, as land-based soldiers specifically trained to engage and defeat the enemy in face-to-face combat; thus, they bear the brunt of the casualties of warfare.
Split into simpler sentences: Infantry is the branch of an army who fight on foot. Historically, infantrymen are land-based soldiers specifically trained to fight on foot and to engage the enemy in face-to-face combat; thus, infantry bear the brunt of the casualties of warfare.