Quran

This is an indication that Allah is being with them does not keep some from becoming martyrs; in fact, that is one of the fruits of being with Allah, as those who are martyred will attain eternal happiness in the Hereafter, and those who remain will attain happiness in this world and the Hereafter. Al-Biqai: 1/279.
Question: Is the fact that Allah is being with patient fighters would keep them from becoming martyrs?

It is well known that the wise individual does not give up what he loves unless it is for something better. Hence, Allah reveals that the whoever is killed for His sake when fighting so that the word of Allah may be supreme and His religion may prevail, not for any other purpose, is not missing out on the life that he loves, rather, he has attained a life that is more perfect than what you may think or imagine. Al-Saadi: 75.
Question: When does a person give up what he loves?

It is said that they were tested as a sign for those who come after them so that they know they were patient as truth was clear to them. Alternatively; It is said that Allah informed them of the coming test so that they would be certain that it would happen, be ready for it and become less likely to be anxious. Moreover, it advances Allah's reward for their resolve and preparation of the soul. Al-Qurtubi: 2/462.
Question: Why does Allah inform His servants that they will be tested?

If times of ease were to persist for people of faith without any trials, then telling truth apart from falsehood would not be clear and that would lead to mischief. Allah's wisdom dictates that good people should be distinguished from others. The purpose of tests is not to cause the believers to lose their faith or turn away from the religion, for Allah will not let the faith of the believers go to waste. But in this verse, He reveals that He will test His servants (with some fear) of their enemies (and famine). Meaning: He will test them with a little of these things, because if He tested them with extensive fear or hunger, they would perish, but the purpose of tests is to purify, not to destroy. Al-Saadi: 76.
Question: Why are people tested with some fear and famine, and not with all?

That is: Allah owns us and we are under His control; we have nothing of ourselves and our property at all. If He afflicts us with any of it, then the Most Merciful has decreed something concerning His servants and their property as He wills, so there should be no objection to Him. Al-Saadi: 76.
Question: It is appropriate for someone who is afflicted with a calamity to say, (We belong to Allah). Why?

We belong to Allah), indicates ownership to Allah, and the owner does in His dominion what He Wills. (And to Him we will return), a reminder of the Hereafter to ease for them this life's afflictions. In the authentic hadith, Allah's Messenger (r) said: "If any servant suffers a calamity and says: "We belong to Allah and to Him we will return; O Allah, reward me for my affliction and replace it with something better," Allah will replace it with something better." Umm Salama said: "When Abu Salama died, I said these words, and Allah replaced him with the Messenger of Allah (r)." Ibn Juzay: 1/89.
Question: What is the recommended supplication to say when a calamity strikes?

Allah made these words a refuge for those afflicted and a protection for those who are tested because of the blessed meanings they contain; namely, Tawhīd (Oneness of Allah), acknowledging His servitude, the resurrection from the graves, and entrusting all matters to Him as they belong to Him. Saeed bin Jubair (a prominent scholar) said: These words were not given to any prophet before our Prophet (r), and if Prophet Jacob (u) had known them, he would not have said: (Oh, my sorrow over Joseph). Ibn Atiyah: 1/228.
Question: What is the wisdom behind saying this supplication during calamities?