I think Enterprise covered it fairly well. They definitely started it more towards the science/exploratory side but quickly ran into the "1000001 ways to die horribly in space" and started jerry-rigging stuff like phasers and photon torpedoes from what was formerly mining equipment etc. Even then they did still have a command structure similar to military, which just makes sense from a practical "who's in charge of what" perspective. They also do have a mix of civilian and more military-style ranks.

Starfleet may have an overall scientific mission/objective, but it definitely developed along the the structure of a military protectorate or coalition, though again that was likely due to a certain amount of necessity. I'm pretty sure that exploratory or trading vessels in historic times tended towards that in some regard.