The alliances that the European powers committed themselves to were defensive in nature. What is meant by this is that they were only triggered if a member of the alliance was attacked by a power identified in the alliance as a likely enemy. For example, in the Dual Alliance between Austria-Hungary and Germany, either country would only support the other if it was attacked by Russia (the ‘named’ country). They were mot made with the intent of attacking an enemy.
But two other things need to be added to this scenario which, when we do, shows how in July, 1914, the alliances, in fact, became aggressive. The first is that with the identification of likely enemies, war plans were made, and these plans were aggressive in nature as they all intended to go on the attack. We should not be totally surprised by this, military chiefs would not be doing their job if their political leaders had identified likely enemies and they hadn’t planned to deal with them. But they were all offensive in nature. They intended to attack the enemy on their territory rather than defend their own territory. So, the consequence was that Germany had a plan, the Schlieffen Plan, to first attack France and quickly defeat it before attacking Russia. Whilst France had its own plan, Plan 17, to hold the expected German offensive and to go on the attack itself – in Alsace and Lorraine, taking back the provinces lost to Germany in 1871. And Russia had its plan too, Plan 19B, mobilising at the same time against both Austria-Hungary and Germany.
And this brings us to the final point to consider: mobilisation. Mobilisation was an incredible logistical challenge: Germany, Austria-Hungary, Russia and France all had millions of men to mobilise. That is they had to get them from civilian life, into barracks and into uniform, and from there, to the fronts. If they failed to do this in time, and their enemy succeeded, the war would most likely be lost in a matter of weeks.
Consequently, in the July Crisis military chiefs were urging their monarchs to act decisively, all of which added immense pressure to decide on war.