With international tension already high in Central Europe after the German annexation of Austria in March 1938 and the continued unrest in the German-speaking border regions of Czechoslovakia, the Sudetenland, reports of substantial military concentrations in areas close to Czechoslovakia on 19 May 1938 gave rise to fears of an imminent German attack. In response to the reports, originating mainly from Czechoslovak intelligence sources, Czechoslovakia mobilised a number of military reservists on 20 May and strengthened its border defences.