Spanish Civil War
On 18 July 1936, war broke out on the Iberian peninsula as Spain dissolved into a state of civil war. After the chaos of the initial uprising, two sides coalesced and began to consolidate their position—the Popular front (the Republicans) and the Spanish Nationalist front. In an early example of a proxy war, both sides quickly received support from other countries, most notably the Soviet Union and Germany, who wanted to test their tactics and equipment. The first shipment of foreign tanks, 50 Soviet T-26s, arrived on 15 October. The shipment was under the surveillance of Nazi Germany's Kriegsmarine and Germany immediately responded by sending 41 Panzer I's to Spain a few days later. This first shipment was followed by four more shipments of Panzer I Ausf. B's, with a total of 122 vehicles.
The first shipment of Panzer I's was brought under the command of Lieutenant Colonel Wilhelm Ritter von Thoma in Gruppe Thoma (also referred to as Panzergruppe Drohne). Gruppe Thoma formed part of Gruppe Imker, the ground formations of the German Condor Legion, who fought on the side of Franco's Nationalists. Between July and October, a rapid Nationalist advance from Seville to Toledo placed them in position to take the Spanish capital, Madrid. The Nationalist advance and the fall of the town of Illescas to Nationalist armies on 18 October 1936 caused the government of the Popular Front's Second Republic, including President Manuel Azaña, to flee to Barcelona and Valencia. In an attempt to stem the Nationalist tide and gain crucial time for Madrid's defence, Soviet armor was deployed south of the city under the command of Colonel Krivoshein before the end of October. 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. This was the first tank battle in the Spanish Civil War. Despite initial success, poor communication between the Soviet Republican armor and Spanish Republican infantry caused the isolation of Captain Arman's force and the subsequent destruction of a number of tanks. This battle also marked the first use of the molotov cocktail against tanks. Ritter von Thoma's Panzer Is fought for the Nationalists only days later on 30 October, and immediately experienced problems. As the Nationalist armor advanced, it was engaged by the Commune de Paris battalion, equipped with Soviet BA-10 armored cars. The 45-millimeter gun in the BA-10 was more than sufficient to knock out the poorly armored Panzer I at ranges below 500 meters (550 yd).
Although the Panzer I would participate in almost every major Nationalist offensive of the war, the Nationalist army began to deploy more and more captured T-26 tanks to offset their disadvantage in protection and firepower. At one point, von Thoma offered up to 500 pesetas for each T-26 captured. Although the Panzer I was initially able to knock out the T-26 at close range—150 meters (165 yd) or less—using an armor-piercing 7.92 millimeter bullet, the Republican tanks began to engage at ranges where they were immune to the machine guns of the Panzer I.
The Panzer I was upgraded in order to increase its lethality. 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-millimeter gun. Ultimately, the piece chosen was the Breda Model 1935, due to the simplicity of the design over competitors such as the German Flak 30. Furthermore, the 20 mm Breda was capable of perforating 40 millimeters of armor at 250 meters (1.57 in at 275 yd), which was more than sufficient to penetrate the frontal armor of the T-26. Although originally 40 Italian CV.35 light tanks were ordered with the Breda in place of their original armament, this order was subsequently canceled after it was thought that the adaptation of the same gun to the Panzer I would yield better results. Prototypes were ready by September 1937 and an order was placed after successful results. The mounting of the Breda in the Panzer I required the original turret to be opened at the top and then extended by a vertical supplement. Four of these tanks were finished at the Armament Factory of Seville, but further production was canceled as it was decided sufficient numbers of Republican T-26 tanks had been captured to fulfill the Nationalist leadership's request for more lethal tanks. 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.
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-millimeter Maklen anti-tank gun, which had been deployed to Asturias in late 1936 on the Soviet ship A. Andreiev. It remains unknown to what extent these trials and adaptations were completed, although it is safe to assume neither adaptation was successful beyond the drawing board.
Second World War
During the initial campaigns of the Second World War, Germany's light tanks, including the Panzer I, formed the bulk of its armored strength. In March 1938, the German Army marched into Austria, experiencing a mechanical breakdown rate of up to thirty percent. However, the experience revealed to Guderian several faults within the German Panzerkorps and he subsequently improved logistical support. In October 1938, Germany occupied Czechoslovakia's Sudetenland, and the remainder of the country in March 1939. 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. It also prepared German forces for the invasion of Poland.
Poland and the campaign in the west
On 1 September 1939, Germany invaded Poland using seventy-two divisions (including 16 reserve infantry divisions in OKH reserves), including seven panzer divisions (1., 2., 3., 4., 5., 10., "Kempf") and four light divisions (1., 2., 3., 4.). Three days later, France and Britain declared war on Germany. The seven panzer and four light divisions were arrayed in five armies, forming two army groups. The battalion strength of the 1st Panzer Division included no less than fourteen Panzer Is, while the other six divisions included thirty-four. A total of about 2,700 tanks were available for the invasion of Poland, but only 310 of the heavier Panzer III and IV tanks were available. Furthermore, 350 were of Czech design—the rest were either Panzer Is or Panzer IIs. The invasion was swift and the last Polish pockets of resistance surrendered on 6 October. The entire campaign had lasted five weeks (with help of the Soviet forces, which attacked on 17 September), and the success of Germany's tanks in the campaign was summed up in response to Hitler on 5 September: when asked if it had been the dive bombers who destroyed a Polish artillery regiment, Guderian replied, "No, our panzers!"
The Poles suffered almost 190,000 casualties (including around 66,300 killed) in the campaign, the Germans around 55,000 (including around 35,000 wounded). However, some 832 tanks (including 320 PzI, 259 PzII, 40 Pz III, 76 PzIV, 77 Pz35(t), 13 PzBef III, 7 PzBef 38(t), 34 other PzBef and some Pz38(t)) were lost during the campaign, approximately 341 of which were never to return to service. This represented about a third of Germany's armor deployed for the Polish campaign. During the campaign, no less than half of Germany's tanks were unavailable due to maintenance issues or enemy action, and of all tanks, the Panzer I proved the most vulnerable to Polish anti-tank weapons.
Furthermore, it was found that the handling of armored forces during the campaign left much to be desired. 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. It was only after Army Group South had its attention taken from Warsaw at the Battle of Bzura that Guderian's armor was fully unleashed. There were still lingering tendencies to reserve Germany's armor, even if in independent divisions, to cover an infantry advance or the flanks of advancing infantry armies. Although tank production was increased to 125 tanks per month after the Polish Campaign, losses forced the Germans to draw further strength from Czech tank designs, and light tanks continued to form the majority of Germany's armored strength.
Months later, Panzer Is participated in Operation Weserübung—the invasion of Denmark and Norway.
Despite its obsolescence, the Panzer I was also used in the invasion of France in May 1940. Of 2,574 tanks available for the campaign, no fewer than 523 were Panzer Is. Furthermore, there were only 627 Panzer IIIs and IVs. At least a fifth of Germany's armor was composed of Panzer Is, while almost four-fifths was light tanks of one type or another, including 955 Panzer II, 106 Czech Panzer 35(t), and 228 Panzer 38(t). For their defense, the French boasted up to 4,000 tanks, including 300 Char B1, armed with a 47-millimeter (1.7 in) gun in the turret and a larger 75-millimeter (2.95 in) low-velocity gun in the hull. The French also had around 250 Somua S-35, widely regarded as one of the best tanks of the period, armed with the same 47 millimeter main gun and protected by almost 55 millimeters (2.17 in) of armor at its thickest point. Nevertheless, the French also deployed over 3,000 light tanks, including about 500 World War I-vintage FT-17s. The two main advantages that German armor enjoyed were radios allowing them to coordinate faster than their British or French counterparts and superior tactical doctrine.
North Africa and campaigns in the east
Italian setbacks in Egypt and their colony of Libya caused Hitler to dispatch aircraft to Sicily, and a blocking force to North Africa. This blocking force was put under the command of Lieutenant General Erwin Rommel and included the motorized 5th Light Division and the 15th Panzer Division. This force landed at Tunis on 12 February 1941. Upon arrival, Rommel had around 150 tanks, about half Panzer III and IV. The rest were Panzer I's and IIs, although the Panzer I was soon replaced. On 6 April 1941, Germany attacked both Yugoslavia and Greece, with fourteen divisions invading Greece from neighboring Bulgaria, which by then had joined the Tripartite Pact. The invasion of Yugoslavia included no less than six panzer divisions, which still fielded the Panzer I. Yugoslavia surrendered 17 April 1941, and Greece fell on 30 April 1941.
The final major campaign in which the Panzer I formed a large portion of the armored strength was Operation Barbarossa, 22 June 1941. The 3,300 German tanks included about 410 Panzer I's. By the end of the month, a large portion of the Red Army found itself trapped in the Minsk pocket, and by 21 September Kiev had fallen, thereby allowing the Germans to concentrate on their ultimate objective, Moscow. Despite the success of Germany's armor in the Soviet Union, between June and September most German officers were shocked to find their tanks were inferior to newer Soviet models, the T-34 and Kliment Voroshilov (KV) series. Army Group North quickly realized that none of the tank guns currently in use by German armor could penetrate the thick armor of the KV-1. 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 suitable for this front. Some less battle-worthy Panzer I's were tasked with towing lorries through mud to alleviate logistics problems at the front.
After Germany, Spain fielded the largest number of Panzer I tanks. A total of 122 were exported to Spain during the Spanish Civil War, and, as late as 1945, Spain's Brunete Armored Division fielded 93. The Panzer I remained in use in Spain until aid arrived from the United States in 1954 when they were replaced by the relatively modern M47 Patton. Between 1935 and 1936, an export version of the Panzer I Ausf. B, named the L.K.B. (Leichte Kampfwagen B), was designed for export to Bulgaria. Modifications included up-gunning to a 20-millimeter gun and fitting a Krupp M 311 V-8 gasoline engine. Although three examples were built, none were exported to Bulgaria, although a single Panzer I Ausf. A had previously been sold. In 1937, around ten Ausf. As were sold to China during a period of Sino-German cooperation until 1941, which were used in the Battle of Nanjing by the 3rd Armored Battalion of the Chinese Nationalist Army to fight against the Japanese Imperial Army. A final order was supplied to Hungary in 1942, totalling eight Ausf. B's and six command versions. These were incorporated into the 1st Armored Division and saw combat in late 1942.