3 Surely he will save you
from the fowler’s snare
and from the deadly pestilence.
4 He will cover you with his feathers,
and under his wings you will find refuge;
his faithfulness will be your shield and rampart.
5 You will not fear the terror of night,
nor the arrow that flies by day,
6 nor the pestilence that stalks in the darkness,
nor the plague that destroys at midday.
7 A thousand may fall at your side,
ten thousand at your right hand,
but it will not come near you.

This is 'a promise from God'. But it clearly doesn't mean that no Christian is ever killed in a war. So we have to ask some harder questions. It's my understanding that God will provide us with the protection that we need to achieve the purposes to which he has called us. This is seen in a couple of instances in Jesus' life when he walks away from crowds apparently about to harm him (Lk 4:14-30, John 8:59). And of course it's not a justification for taking unnecessary risks.