On Christmas Day, 1914, the guns of the Western Front suddenly fell silent. All along the line, tens of thousands of soldiers lay down their weapons, climbed out of their trenches, and wandered into no-man’s land. Men who just hours ago had been actively trying to kill each other suddenly began fraternizing like old friends, exchanging food and gifts, recovering and burying each others’ dead, singing Christmas carols, and even playing football matches. But as the sun came up the following morning, the troops returned to their trenches and the shelling and fighting resumed once more. The Christmas Truce of 1914 has become the stuff of legend, a heartwarming display of humanity in the midst of industrialized slaughter. But just one year later, an even more bizarre event took place as the German and British Empires, desperately short on vital wartime resources, turned, astonishingly, to each other. This is the forgotten story of the great Glass-for-Rubber trade, one of the most surreal episodes of the Great War.

The Great War saw the combat debut of many deadly new weapons, including aircraft, submarines, tanks, and poison gas. But the true king of the battlefield was still artillery, which killed more soldiers throughout the war than any other weapon. First World War artillery pieces could lob high explosive, shrapnel, and chemical shells up to 10 kilometres away – beyond the limits of human vision. Achieving accurate fire thus required the use of precision gunsights, rangefinders, and other instruments. Binoculars, trench periscopes, and airborne reconnaissance cameras were also needed in large quantities, making high-quality optical glass a vital strategic resource. For most of the 19th Century, much of the world’s optical glass had come from two companies: Chance Brothers of Birmingham, England, and Parra Mantois et Compagnie of Paris, France. However, in the 1880s a group of glassmakers based in the German city of Jena including Carl Zeiss, Ernst Abbe, and Otto Schott revolutionized the optics industry by creating Jena Glass, which was clearer and more heat and shock-resistant than anything that had come before. Almost overnight, Germany became the world leader in high-quality optics, exporting nearly 176,400 kilograms of optical glass per year by 1912. By 1914, 60% of the British Empire’s military optics were made in Germany, with 30% coming from France and only 10% produced domestically. With the outbreak of war, Britain suddenly found itself cut off from its biggest glass supplier. Meanwhile, Parra Mantois, struggling to meet the requirements of the French Army, had no leftover capacity to meet Britain’s needs. The British armed forces thus found themselves facing an optics shortage of disastrous proportions.

Part of the problem lay in the British Government’s lack of foresight. In the early months of the War, Britain’ largest supplier of optical glass, Chance Brothers, insisted that its existing production facilities would be more than sufficient to meet the nation’s military needs. Given that at the time most military planners believed the War would be over by Christmas, this was not an unreasonable assumption. By early 1915, however, it became clear that the war would likely drag on far longer than anyone had predicted, and that Britain’s demand for optical glass would far exceed its domestic production capacity. As a stopgap measure, the British Government called on its citizens to donate their own binoculars and telescopes to the war effort, and commandeered unsold optics from private manufacturers and retailers. But this was hardly enough to feed the hungry machine, and in late 1915 the newly-created Ministry of Munitions, headed by future Prime Minister David Lloyd George, proposed a radical – and seemingly treasonous – solution: buy the required optics from Germany.

Incredibly, the proposal was accepted, and, working through intermediaries in neutral Switzerland, the Ministry succeeded in brokering a deal whereby German firms like Zeiss would supply the British military with 32,000 pairs of binoculars – 20,000 high-quality models for artillery officers and 12,000 lower-quality models for regular officers. In return, the German government asked for large quantities of rubber from Britain’s colonies in the Far East. Just like glass, modern mechanized warfare had turned rubber into a strategically-vital resource, with vast quantities of the substance being needed to make truck and bicycle tyres, gas masks, and other important equipment. At the outbreak of war, Germany’s overseas colonies were captured by the Entente powers and its ports blockaded by the Royal Navy, cutting off all its regular rubber supplies. In response, the German government implemented a program of rubber recycling known as Kautschuk-Regenerat and funded research into synthetic rubber substitutes. Both German tyre manufacturer Continental and pharmaceutical giant Bayer produced truck tyres made of dimethylbutadiene or “methyl rubber”, but these were brittle and had to be replaced after only 2,000 kilometres. Furthermore, the limited production capacity of both companies could not hope to meet Germany’s military demands. So dire was this rubber shortage that by 1917 large numbers of German Army trucks were being fitted with steel wheels that tore up roads, while cyclists had to contend with bulky metal leaf-spring tyres that did little to smooth the ride. It is thus perhaps less surprising that the Germans were willing to consider trading with the enemy.

Incredibly, this deal between belligerent nations was not, strictly speaking, illegal. While the British Trading With The Enemy Act of 1914 ruled that all German assets in Britain were to be seized and placed in the common trust, it said nothing about trade through neutral third parties. However, as the Glass-for-Rubber trade would have looked more than a little treasonous to the people of both nations, the negotiations and details of the deal were understandably kept top-secret. Unfortunately, one result of this secrecy is that few official records of this deal survive, meaning we have no idea what quantities of goods were actually exchanged or when. But while this has led some historians to suspect that the actual trade never took place, this would not be the last time the Entente and Central Powers would do business during the war. Just one year later, for example the British bought a large quantity of German-manufactured aircraft engine magnetos – again, though a Swiss intermediary. And in 1918, the Germans promised the new Bolshevik government of Russia to defend the oil fields of Baku against the Ottoman Empire – then Germany’s ally – in exchange for oil from said fields.

Yet even if the deal actually did go though, it would not have been enough to meet Britain’s ever-growing wartime demand for glass. Thus, in June 1915, the Ministry of Munitions struck a deal with Chance Brothers whereby the government would fund the expansion of the company’s production facilities in exchange for manufacturing a large batch of military optics. The deal was highly productive for both parties, and while Britain never achieved complete self-sufficiency in optical glass, the injection of government funding allowed Chance Brothers to achieve a level of quality rivalling that of pre-war German firms.

But trade deals between enemies are far from unique to the First World War. 30 years later, Nazi Germany would propose a similar exchange – though this time the circumstances were considerably darker. In March 1944, the Nazis launched Operation Margarethe, the invasion and occupation of the Kingdom of Hungary. Hungary had entered the Second World War allied with Germany, being the fourth nation to join the Axis Powers after Italy and Japan. Hungarian troops fought alongside German forces during the invasion of Yugoslavia and the Soviet Union, but following a series of costly defeats, Hungarian regent Admiral Miklós Horthy sought to distance himself from the Nazis. In March 1942, Horthy replaced pro-German Prime Minister László Bárdossy with the anti-fascist Miklós Kállay, who in 1944 negotiated an armistice with the Allies and withdrew Hungary from the Axis. In response, Hitler ordered German troops to occupy Hungary and depose Horthy and Kállay. In the course of this occupation, German forces rounded up more than 437,000 Hungarian Jews, who until that point had largely been spared from the Nazi final solution. These people were slated to be transported to Auschwitz for extermination, but in April 1944 SS-Obersturmbannführer Adolf Eichmann, head of the Reich Security Head Office Jewish Affairs Department, decided instead to use them as bargaining chips. By this point in the war, relentless Allied aerial bombardment had severely crippled Germany’s industrial base, leaving the German military desperately short of motor vehicles. Eichmann thus proposed trading one million Hungarian Jews for 10,000 trucks from Britain or the United States. The Jews would be allowed to emigrate to any Allied-held territory except Palestine, which the Nazis had promised to muslim Grand Mufti of Jerusalem Hajj Amin Al-Husseini. To broker this deal, dubbed Blut gegen Waren or “blood for goods,” Eichmann approached Brand Jenö and Andor Grosz, members of the Budapest Aid and Rescue Committee or Va’da, an underground group which had previously smuggled Jews from occupied Europe to the relative safety of Hungary.

However, the Allies never took the proposal seriously, considering the trade a “monstrous offer” and suspecting the deal to be a trick, designed to win the Nazis a separate peace with the Western Allies. There were also more practical – and cynical – reasons for refusing the offer, such British reluctance to absorb such a large number of Jewish refugees. In the end, all but 15,000 of the 437,000 Hungarian Jews rounded up in 1944 were deported to Auschwitz, where 90% were killed on arrival. In total, 544,500 of Hungary’s jews were murdered by the Nazis during the Second World War – third only to Poland and the Soviet Union.