From the creation of Adam as the Khalifa, Allah started to wait for the moment when any human would fulfill the honor of Khilafat as bestowed by Allah Almighty. Someone had to come to prove to the angels that God created man and honored him with Khilafat and he deserves it.

The exam of Adam in Jannah not to come close to that tree. As in Jannat exam could not have been of thirst and hunger because Jannat has food and water all the time. Yet he could not fulfill the exam. Put ref.

The exam of Ibrahim in fire. Ibrahim could not bear the heat so Allah said oh fire become safe and cool for Ibrahim.

The exam of Ismael when he was young, he could not bear thirst so Allah granted Zamzam.

The exam of Ismael when Ibrahim had to sacrifice him but Allah sent a goat instead so slaughter of Ismael was saved.

The exam of Yaqoob so he eventually met Yousuf.

The exam of Bibi Maryam so Allah granted Essa the power to speak and hence Maryam did not have to defend herself and was saved from humiliation.

All of the prophets had to undergo a trial and all of them asked Allah’s help at one point or the other. And then Allah helped them and their exam was over.

Now in the case of Karbala. Imam Hussain started giving from 7th of Muharram till nothing was left to ask from him. He never asked Allah’s help to end the exam during the exam.