Qur'an 57:25

We have already sent Our messengers with clear evidences and sent down with them the Scripture and the balance that the people may maintain [their affairs] in justice. And We sent down iron, wherein is great military might and benefits for the people, and so that God may make evident those who support Him and His messengers unseen. Indeed, God is Powerful and Exalted in Might. (Qur'an 57:25)

I was just reading that Sura today and stopped to reflect at that verse I don't remember ever really noticing it before. So God sent iron (and thus, weapons) to the world to show who supports Him and his messengers, as those who do so will use weapons to fight to establish justice. It reminded me of the story of Moses and Pharaoh and how Pharaoh declared himself a god, had all the worldly power and demanded Moses to submit to him; whereas Moses submitted only to God and had none but truth and justice on his side. If we take up weapons we have to choose which side we're on, the side of the tyrants who have the worldly power or that of the Prophets (which in these times is the side of truth and justice). If we are believers we will choose the later even if it has little worldly power with it and will likely result in us giving up our lives. Otherwise we would be worshipping worldly power like Pharaoh demanded people worship him as a God. But there is no god but God, and the martyrs are His Beloved.