Kaiser Wilhelm II and the blank cheque

What specific decision made Wilhelm II responsible for escalating the crisis?

His blank cheque to Austria-Hungary encouraged escalation; withholding it might have left more room for a conference and negotiation.

The explanation centres on Kaiser Wilhelm II's blank cheque to Austria-Hungary. Without that commitment, Austrian leaders are judged more likely to have taken a conservative approach. A conference and negotiated settlement are offered as possible outcomes of that alternative. This is a counterfactual argument about the consequences of German support, rather than proof that a conference would have prevented war.