The Germans had issued an ultimatum to Russia, essentially saying that if Russia did not halt mobilization against Austria, then Germany would declare war. This ultimatum expired at 5:00 pm on August 1st, 1914.

In Berlin on August 1st, the crowds milling about the streets and massed in thousands in front of the palace were tense and heavy with anxiety ... At 5 o'clock a policeman appeared at the palace gate and announced mobilization to the crowd, which obediently struck up the national hymn ... Cars raced down Under den Linden with officers standing up in them, waving handkerchiefs and shouting, "Mobilization!" (From The Guns of August by Barbara Tuchman)

The Germans at the same time issued an ultimatum to France, asking for their neutrality in the event of German conflict with Russia (which would break France's treaty obligations) and also for German troops to occupy the French fortress at Verdun for 'the duration of the war'. This was obviously unacceptable, and so Germany went to war with France as well. Their battle plan called for a first strike on France by way of Belgium and Luxembourg. Belgium was also issued an ultimatum asking for safe passage for German troops on their way to France, which was refused.

All these ultimatums were issued as a way to legitimize the German war plan in the eyes of the public and of the world. The German people would have been aware of the major troop movements and the countries that were arrayed against the Central Powers.