Pakistan, Aug. 9 -- At Senator Lindsey Graham's funeral, President Trump said Graham "never saw a war that he didn't like." It was a fitting epitaph for a man who, before his death, had already dismissed Pakistan's peacemaking efforts between the United States and Iran as "problematic." Now that the ceasefire has crumbled, critics are circling the same question: Did Pakistan fail? But that is the wrong question. The right one is far more uncomfortable. Why do we expect a mediator to guarantee a peace that the warring parties themselves are not ready to keep?

To appreciate how unfair that expectation is, consider how rarely peace has been guaranteed even by the most powerful actors on the world stage. After the devastation of World War I,...