How did Constantius II’s death prevent a civil war with Julian?

In 361, Julian—who had served as Caesar in Gaul since 355—was proclaimed Augustus by his troops, directly challenging the authority of Constantius II. This act of usurpation set the two leaders on a collision course, as Constantius II left his military campaigns in the East to confront Julian in the West.12

However, a full-scale civil war was averted because Constantius II fell ill and died in November 361 while traveling to meet Julian’s forces. His death left the empire to Julian, who became the sole ruler and the final member of the Constantinian family to hold the throne.12