During the following year, Greece's official governments were hard-pressed to maintain the country's neutrality. The final straw came when, on , the Athens government, succumbing to German pressure, ordered the surrender of the vital Rupel Fortress to the Germans and their Bulgarian Allies. In response, on , the pro-Entente Venizelists imposed martial law, effectively abolishing royal sovereignty in all of northern Greece. On , the Bulgarian invasion of eastern Macedonia commenced, facing little resistance, since the Athens government refused to condone any firm action. As a result, more than 6000 men of IV Corps surrendered to the Germans on and were deported to Görlitz in Germany. This surrender of hard-won territories with only token resistance, outraged most Greeks. At the same time, the establishment of the exiled Serbian King and his government in Thessaloniki in April, the presence of 120,000 Serbian troops in the Macedonian Front, accompanied by threats from the Entente that he would install a Serbian prefect in the city, raised fears that the city would be handed over to the Serbians.