Infantry is the branch of an army that fights on foot. Infantrymen are land-based soldiers who are specifically trained for the role of fighting on foot to engage the enemy face-to-face and have historically borne the brunt of the casualties of combat in wars. As the oldest branch of combat arms, they are still the backbone of modern armies. Infantry units have more physically-demanding training than other branches of armies, and place a greater emphasis on discipline, physical strength, fitness and spontaneous sustained aggression. The infantryman himself, with or without his personal weapon, is considered a weapon system.
Infantrymen are easily distinguished from soldiers trained to fight on horseback (cavalry), in tanks, or in technical roles such as armourers or signallers. Rudimentary infantry skills such as basic individual movement techniques, shooting positions and field craft are fundamental to the training of every soldier. Infantry can access and maneuver in terrain inaccessible to vehicles and tanks, and employ infantry support weapons that can provide firepower in the absence of artillery. Their combat insertion techniques include, airborne, air assault, amphibious and by land.
Since the end of the Second World War, the infantry has become a very small part of the Western world's armies. Typically between 5% and 30% of an army's personnel are trained infantry. Despite this number they may still represent one of the largest individual arms; with the exception of logistics and supply. Infantry numbers are vastly reduced from pre-WWII levels. For instance, in the United States Army of 2009, there were only approximately 49,000 Infantrymen out of about 565,000 active duty enlisted personnel. The Marine Corps has another 10,000-20,000 Infantrymen. This means that there are fewer than 70,000 Infantrymen in the entire US military. The US Air Force and Navy do not have infantry assets. The Russian Navy on the other hand employs a small number of troops with less than 2,000, trained as Naval Infantrymen. This Naval Infantry functions similar to the US Marine Infantry which traditionally assault land objectives from water born assault platforms.
These lower infantry numbers reflect the greatly increased lethality, degree and type of training afforded to modern Infantry regiments. Western armies have increasingly complex technology based weapons systems which increases the technical and logistics demand on infantrymen in Western armies.