Reinsurance Treaty
The "elaborate alliance systems" among European powers, "each secured by a network of secret treaties, financial arrangements, and 'military understandings'" are commonly cited as one of the causes of World War I. For example, the Reinsurance Treaty of June 1887 between the German Empire and the Russian Empire (negotiated by German Chancellor Otto von Bismarck in order for Germany to avoid a two-front war), was a "highly secret treaty" in which the two powers pledged a three-year period to remain neutral should the other become involved in a war with a third country, unless Germany attacked Russia's longstanding ally France or Russia attacked Germany's longstanding ally Austria-Hungary.