Facing what it believed to be intolerable pressures, Germany declared war on Russia on 1 August 1914. The local Balkan war between Austria-Hungary and Serbia now became European.
The German Ambassador to Russia. St. Petersburg, 1 August 1914.
The Imperial German Government have used every effort since the beginning of the crisis to bring about a peaceful settlement. In compliance with a wish expressed to him by His Majesty the Emperor of Russia, the German Emperor had undertaken, in concert with Great Britain, the part of mediator between the Cabinets of Vienna and St. Petersburg; but Russia, without waiting for any result, proceeded to a general mobilisation of her forces both on land and sea.
In consequence of this threatening step, which was not justified by any military proceedings on the part of Germany, the German Empire was faced by a grave and imminent danger. If the German Government had failed to guard against this peril, they would have compromised the safety and the very existence of Germany.
The German Government was, therefore, obliged to make representations to the Government of His Majesty the Emperor of All the Russias and to insist upon a cessation of the aforesaid military acts. Russia having refused to comply with this demand, and having shown by this refusal that her action was directed against Germany, I have the honour, on the instructions of my Government, to inform your Excellency as follows:
His Majesty the Emperor, my august Sovereign, in the name of the German
Empire, accepts the challenge, and considers himself at war with Russia.
Source: German Documents collected by Karl Kautsky