Animals often perceive things that we don't. They are known to have a much more heightened senses of smell and sight than we do. These are traits that are useful in combat situations where tension is running high and human soldiers are more likely to make mistakes as a result.
No, the military should not use animals in combat situations. That is animal cruelty. Animals should not be used in situations that they know nothing about. I'm sure we could develop some sort of underwater drone or root technology that would be more effective than animals. This would be much more ethical.
No, animals should not participate in combat situations because it is dangerous and they could get killed. Dolphins can help military in normal, everyday situations, but participating in combats is just not acceptable. Dolphins are intelligent, but not intelligent enough to avoid death when being an active participant in a combat. Also, they can easily get frightened and thus useless in combats.
No, the military should not use animals in combat situations because the animals do not have a chance to agree or disagree with fighting. Why should their lives be risked for the sake of a manmade war? They should be able to live freely, without fear of manmade combats in the future.