Mobile armor was, however, an expensive investment. Towed anti-tank guns were cheaper and dominated most armies as a default solution for enemy tanks and became organic parts of large units like corps, divisions and regiments. Standard practice was to place these anti-tank guns at the front line, spread out to ensure full coverage. Nevertheless, experience showed that neither infantry, anti-tank guns, or tanks, when used statically could withstand the deep envelopment maneuvers of armored "spearheads". While some anti-tank guns or tanks could help defend the area the enemy chose to attack "en masse", they could never be enough to prevent the inevitable breakthrough. Moreover, anti-tank guns were vulnerable to infantry and artillery attacks, and even attacks from the tanks they were targeting.