Experiences with the Panzer I during the Spanish Civil War helped shape the German Panzerwaffe's invasion of Poland in 1939 and France in 1940.

Although lacking in armored combat as a tank, it formed a large part of Germany's mechanized forces and was used in all major campaigns between September 1939 and December 1941, where it still performed much useful service against entrenched infantry and other "soft" targets, which were unable to respond even against thin armor, and who were highly vulnerable to machine gun fire.

Paragraph Twenty-four of the treaty provided for a 100,000-mark fine and imprisonment of up to six months for anybody who "[manufactured] armored vehicles, tanks or similar machines, which may be turned to military use".

[4] Despite the manpower and technical limitations imposed on the German Army by the Treaty of Versailles, several Reichswehr officers established a clandestine general staff to study World War I and develop future strategies and tactics.

[5] There was also minor military cooperation with Sweden, including the extraction of technical data that proved invaluable to early German tank design.

[6] As early as 1926 the German companies Krupp, Rheinmetall and Daimler-Benz were contracted to develop prototype tanks armed with a 75 mm cannon.

[7] In the late 1920s and early 1930s, German tank theory was pioneered by two figures: General Oswald Lutz and his chief of staff, Lieutenant Colonel Heinz Guderian.

[9] Like his contemporary, Sir Percy Hobart, Guderian initially envisioned an armored corps (panzerkorps) composed of several types of tanks.

Lastly, Germany needed a heavy tank, armed with a 150 mm (5.9 in) cannon to defeat enemy fortifications, and even stronger armor.

[14] Machine guns were known to be largely useless against even the lightest tank armor of the time, restricting the Panzer  I to a training and anti-infantry role by design.

The tank had several design flaws, including suspension problems, which made the vehicle pitch at high velocities, and engine overheating.

The larger engine required the extension of the vehicle's chassis by 40 cm (16 in), and this allowed the improvement of the tank's suspension, adding another bogie wheel and raising the tensioner.

147 more training tanks were built as convertible chassis with hardened armor with the option to upgrade them to full combat status by adding a superstructure and turret.

By this stage, the design concept had been superseded by medium and heavy tanks and neither variant was produced in sufficient numbers to have a real impact on the progress of the war.

In an early example of a proxy war, both sides quickly received support from other countries, most notably the Soviet Union and Germany as both wanted to test their tactics and equipment.

[36] Between July and October, a rapid Nationalist advance from Seville to Toledo placed them in position to take the Spanish capital, Madrid.

[38] In an attempt to stem the Nationalist tide and gain crucial time for Madrid's defense, Soviet armor was deployed south of the city under the command of Colonel Krivoshein before the end of October.

[39] At this time, several T-26 tanks under the command of Captain Paul Arman were thrown into a Republican counterattack directed towards the town of Torrejon de Velasco in an attempt to cut off the Nationalist advance north.

On 8 August 1937, Major General García Pallasar received a note from Generalísimo Francisco Franco that expressed the need for a Panzer I (or negrillo, as their Spanish crews called them) with a 20 mm gun.

The Breda modification was not particularly liked by German crews, as the unprotected gap in the turret, designed to allow the tank's commander to aim, was found to be a dangerous weak point.

[48] In late 1938, another Panzer I was sent to the Armament Factory of Seville in order to mount a 45 mm gun, captured from a Soviet tank (a T-26 or BT-5).

A second was sent sometime later in order to exchange the original armament for a 37 mm Maklen anti-tank gun, which had been deployed to Asturias in late 1936 on the Soviet ship A. Andreiev.

[citation needed] During the initial campaigns of World War II, Germany's light tanks, including the Panzer I, formed the bulk of its armored strength.

The capture of Czechoslovakia allowed several Czech tank designs, such as the Panzer 38(t), and their subsequent variants and production, to be incorporated into the German Army's strength.

During the beginning of Guderian's attack in northern Poland, his corps was held back to coordinate with infantry for quite a while, preventing a faster advance.

[67] Setbacks in the Italian invasion of Egypt caused Hitler to dispatch aircraft to Sicily, and a blocking force (the Afrika Korps) to support their ally in the North Africa campaign.

[74] The final major campaign in which the Panzer I formed a large portion of the armored strength was Operation Barbarossa, 22 June 1941.

[75] By the end of the month, a large portion of the Red Army found itself trapped in the Minsk pocket,[76] and by 21 September Kiev had fallen, thereby allowing the Germans to concentrate on their ultimate objective, Moscow.

[78] The performance of the Red Army during the Battle of Moscow and the growing numbers of new Soviet tanks made it obvious the Panzer I was not largely suitable for this front of war.

[79] Some less battle-worthy Panzer Is were tasked with towing lorries and other light (mainly wheeled) vehicles through the thick mud of the Russian autumn to alleviate logistical and transportation issues and problems at the frontlines,[79] whilst other Panzer Is were relegated for anti-partisan actions or rear-guard protection duties (such as defending airfields or other vital military installations on occupied enemy territory).