Self-propelled artillery
They are high-mobility vehicles, usually based on continuous tracks carrying either a large field gun, howitzer, mortar, or some form of rocket/missile launcher.
Modern self-propelled artillery vehicles often mount their main gun in a turret on a tracked chassis so they superficially resemble tanks.
The gunners were trained to quickly dismount, deploy the guns and provide instant fire support to cavalry, and act as a flexible reserve.
The Russian army organized small units of horse artillery that were distributed among their cavalry formations in the early 18th century.
While not forming large batteries and employing only lighter 2- and 3-pound guns, they were still effective and inflicted serious losses to Prussian units in the Seven Years' War.
Other nations quickly realized the capability of the new arm and by the start of French Revolutionary Wars in 1790s Austria, Hannover, Portugal, Russia, France, Great Britain and Sweden had all formed regular units of horse artillery.
In effect, the carrier replaced the use of a separate horse team or internal combustion engine-powered artillery tractor, and allowed a new way for the gun to be used.
This mounted an 18-pounder field gun, capable of both the usual artillery trajectories and high-angle anti-aircraft fire, on a Vickers medium tank chassis.
It was designed and built for investigations into a general approach to warfare where all arms, infantry and artillery included, would be able to operate over the same terrain as tanks.
While the German Blitzkrieg doctrine called for combined-arms action, which required fire support for armoured units, during the invasion of Poland and France this was provided by the Luftwaffe using Junkers Ju 87 'Stuka' dive-bombers effectively acting as artillery.
For example, the first British design, "Bishop", carried the 25 pdr gun-howitzer on a Valentine tank chassis, but in a mounting that severely limited the gun's performance.
The Soviets experimented with truck- and tank-based self-propelled weapons, producing a few Komsomolets tractor-mounted 57 mm ZiS-2 guns early in the war.
These had the advantage of being relatively cheap to build and mounting a larger gun compared to the conventional tank that they were derived from, but at the expense of flexibility.
Although often similar to tank destroyers, they carried larger-caliber guns with weaker anti-armor performance but capable of firing powerful HE projectiles.
All major nations developed self-propelled artillery that would be able to provide indirect support while keeping pace with advancing armoured formations.
These were usually lightly armoured vehicles with an open-topped hull; the American M7 Priest, the British Sexton (25 pdr) and the German Wespe and Hummel being typical examples.
The Israeli Merkava main battle tank carried a 60 mm mortar in the small troop compartment in the rear, which fired through an opening in the roof, allowing the crew to remain protected.
These capabilities also increase survivability manyfold as modern SP artillery can displace and avoid counterbattery fire much more quickly and effectively and, if desired, more frequently than previously possible.
