During World War I, a conference took place between the German emperor Wilhelm II and the Austro-Hungarian monarch Charles I in Spa on 12 May 1918.^[1] At his meeting, Charles I and his minister Stephan Burián von Rajecz were forced to accept the political and economic subjection of Austria-Hungary to the German Empire in the form of a treaty. Formally concluded on an equal footing between the signatory powers, the agreements reached at Spa in fact endorsed the pre-eminence of Germany and guaranteed its supremacy,^[2] while the Austro-Hungarians were forced into a situation of political, economic and military dependence.^[3] However, the Spa agreement, which made the dual monarchy subject to an "Austro-German Zollverein" (customs union),^[3] failed to put an end to rivalries in the Balkans and Eastern Europe, or to political disagreements over the end of the conflict or the future of occupied Poland.^[4] At the meeting on May 12, German and Austro-Hungarian negotiators agreed to set up technical commissions to put into practice the economic and commercial provisions of the agreement in principle between the emperors.^[2] The subsequent Salzburg negotiations, however, fell apart in October with the imminent defeat of Germany and Austria-Hungary.