The diplomatic crisis grew to a climax as Mackensen's forces reached the northern bank of the Danube, for the arrest of the German offensive in Russia was not entirely due to German difficulties or Russian valour and strategy, and by the middle of August German divisions were already being diverted from the Russian front. In the middle of September Bulgaria concluded her compact with Turkey, and on the 19th Mackensen's batteries opened their bombardment of Belgrade. On the 21st Venizelos asked the Western Allies for 150,000 troops, which were promised on the 24th, and on the 23rd Bulgaria ordered a general mobilization and Greece retorted in kind. Bulgaria proclaimed her intention to observe neutrality, and when on the 27th Serbia requested the consent of the Allies to an offensive, it was refused. Entente diplomatists at Sofia were still under the impression that Bulgarian intervention could be avoided, and a vigorous protest by the leaders of all the Opposition parties in the Sobranje against the Government's policy gave some colour to their views. But by 10 October it became known that many German officers were busy in consultation with the Bulgarian Staff; on the 3rd Russia required Bulgaria to break with the Teutonic Powers, and on the 5th herself broke off diplomatic relations. A week later the Bulgarian Army invaded Serbia and the Bulgarian Government declared war. Mackensen had crossed the Danube on the 7th and taken Belgrade on the 9th. On the 19th an imperial manifesto was issued from Petrograd denouncing Bulgaria's treason to the Slav cause and leaving the fate of the traitor to the "just punishment of God." It was assuredly not to be inflicted by the Government whose designs on Constantinople had been the principal obstacle to the success of Entente diplomacy in the Balkans. Bulgaria did, indeed, betray the Slav to the Teuton, but no Balkan State could view with equanimity the prospect of being ground to powder between the upper and nether millstone of Russia on the Danube and on the Dardanelles.