Complex sentence: Infantry is the branch of a military force that fights on foot--troops specifically trained to engage, fight, and defeat the enemy in face-to-face combat, and thus bear the brunt of warfare, suffering the greatest number of casualties.
Split into simpler sentences: Infantry is the branch of a military force that fights on foot. Troops intended to engage, fight, and defeat the enemy in face-to-face combat, thus bear the brunt of warfare and suffering the greatest number of casualties.