Question: Which intergovernmental military alliance signed in 1949 are Denmark and the Netherlands both members of? 
Sentence (from "Austro–Serbian Alliance of 1881"): The treaty came after the railway convention of 6 April 1881 for the construction of the Belgrade–Niš section of the Vienna–Constantinople railway, and the trade treaty of 6 May 1881 which made Austria-Hungary virtually the sole market for agricultural products from Serbia and thereby dominant.
Relevant evidence: No