Germany was almost entirely dependent on Hungary and Yugoslavia for bauxite, used in the production of Duralumin, a copper alloy of aluminium critical to aircraft production. The British attempted to stop the bauxite trade by sending undercover agents to blast the Iron Gate, the narrow gorge where the Danube cuts through the Carpathian Mountains by sailing a fleet of dynamite barges down the river, but the plan was prevented by Romanian police acting on a tip-off from the pro-German Iron Guard. Despite their declared neutrality, the politically unstable Balkan nations found themselves in an uncomfortable position, surrounded by Germany to the North, Italy to the West and Soviet Union to the East, with little room to refuse German veiled threats that, unless they continued to supply what was requested, they would suffer the same fate as Poland. Romania, which had made considerable territorial gains after World War I, exported a large proportion of the oil from its Ploiești site to Britain, its main guarantor of national sovereignty. Romania's production was about equal to that of Ohio, ranked 16th producer in the US, then a major oil-producing nation. The largest refinery, Astra Română, processed two million tons of petroleum a year but, as Britain's fortunes waned from the beginning of 1940, Romania turned to Germany using its oil as a bargaining tool, hoping for protection from Soviet Union. On 29 May 1940 it stopped sending its oil to Britain, and signed an arms and oil pact with Germany; Romania was soon providing half her oil needs. Britain was able to arrange alternative supplies with the Anglo-Iranian Oil Agreement, signed on 28 August 1940.