Since the days of Napoleon, the mobile artillery piece has been utilized to deadly effect. Artillery became the focal point of many-an-offensive during the days of World War 1 and World War 2 and, since then, base accuracy, range and construction materials have been considerably improved to the point that many world powers can field some type of excellent 155mm field gun design to support infantry and armored force actions. Systems on order heading into 2015 are NOT taken into account.
Towed Artillery strength data through 2014.
Sources: CIA.gov, CIA World Factbook, wikipedia.com, public domain print and media sources and user contributions. Some values may be estimated when official sources are lacking.