Since 1912 German military chiefs had been advocating for war against Russia before their armies grew too strong
Germany, despite this, responded to Austria-Hungary on 5th July with a promise that ‘Austria can rest assured that His Majesty will faithfully stand by Austria Hungary as is required by the obligations of its alliance’
Russia was unlikely to allow its client state, Serbia, to be attacked without responding, and Russia’s involvement would likely prompt French and perhaps British involvement
This promise made by Germany became known as a ‘blank cheque’ as it is implied that Germany would back Austria-Hungary in whatever actions the Empire chose to take