In 1 Samuel 4:5-10, when the Israelites bring the Ark of the Covenant into battle against the Philistines, the Philistines become terrified, believing "a god has come into the camp," and fearing the power of the Israelite God; despite this fear, the Philistines fight and decisively defeat the Israelites, capturing the Ark of the Covenant, causing great panic and loss of life among the Israelites.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.