Let's just take a look at the whole verses and NOT A PART OF IT shall we:
8:12-13 Recall that your Lord inspired the angels: "I am with you; so support those who believed. I will throw terror into the hearts of those who disbelieved. You may strike them above the necks, and you may strike even every finger." This is what they have justly incurred by FIGHTING God and His messenger. For those who fight against God and His messenger, God's retribution is severe.
That is the context of the verse in the Qur'an. The historical context is that this verse was revealed at the Battle of Badr, a battle in which the pagans of Makkah traveled over 200 miles to destroy the Muslims of Madinah. The Pagans of Makkah had an army of about 1000 while the Muslims were only 300 followers. The Prophet Muhammad ((peace be upon him)) and his followers had suffered severe persecutions and torture for 13 years in the city of Makkah. Having fled from Makkah to the safety of Madinah, they found that they were once again threatened.That's when this verse was revealed to boost up the confidences of the muhajir.
Believe me,anyone would need bucking up when they are 300(with no proper weapons or anything) fighting against an army of 1000.